Revived the dot subcommand and the dot printer for 'ls'.

Use this in the configuration file:

[columns]
column_width = 42

PrettyPrinterColorFilter mistakenly overwrote any colors at the
beginning with the priority color.

When doing a 'dep add' command, the parent lookup dictionary was not kept up to
date. Therefore there was an edge in the dependengy graph that did not exist in
the parent dict. When the child item is completed, then its dependency ID is
looked up, but it doesn't exist, resulting in a crash.

By adding the _add_edge method, it should be easier to keep the edges of the
graph and the content of the dictionary in sync.

Relative dates and dependencies in append cmd

This commit adds support for relative dates and
dependencies in the append command. Relative dates
in due and start dates will be automatically converted
and dependenices will be recomputed.

When appending due/start dates, if the command already
has due or start dates, it will be replaced.

For each UI, create a folder in the ui subdirectory.

Conflicts:
	.travis.yml
	setup.py

Similar to the functionality in the dep subcommand, introduced in commit
4ed21654.

Otherwise the wrong start date is calculated when advancing a recurring
todo item.

The progress color will still use a length of 1 for calculating the
appropriate color.

These operators copy the parents of one todo item to another:

    dep add 1 parents-of 2

This will add the parents of item 2 to item 1.

Likewise, a children-of operator adds the children of item 2 to item 1.

Along the way, the _get_todos() method was refactored to simplify things
a bit.

The crash occurs when using text-based identifiers and performing an edit on
selected items (instead of the whole todo.txt file).

If you edit the text of a child item (having a p tag), then the item is deleted
using the TodoListBase.delete() instead of TodoList.delete(). But
this messes up the dependency administration, still thinking that the removed
todo item is still in the list. When you call the children of any todo item
(part of the deletion/completion step), it will return a stale Todo item, whose
number() can not be found anymore in the TodoListBase. That makes it crash.

Don't crash on shlex.split in column UI

Also don't try to do anything after such error occurs so we don't
trigger another crash if default_command is assinged to some alias.

Hopefully finaly fixes #107

Provide adequate "mono" palette for column ui

Focused items are now properly highlighted when colors are set to 0 and
marked items are made bold and underlined.

A weekly recurring task with the start date one day before the due date,
shouldn't be colored orange/yellow when the task starts, but green.

Moreover, items where start date == due_date should have a length of 1.

Without any colors, TopydoString doesn't have any colors set resulting
in an empty markup item. That made the urwid library trip.

Also, disable colors when not connected to a TTY. Before, when the
configuration file configured 256 colors, it would be used whether
regardless of being connected to a TTY.

This is achieved using abstract colors for projects, contexts, etc.. The
Color filter for the pretty printer has no knowledge of colors, it just
marks a certain text as 'project color'. All this information is stored
in the TopydoString. The CLI will convert the colored parts in the
string with the corresponding ANSI codes, the column UI will convert the
string to an appropriate markup for the Text widget.

Also, palette items are now specified using an enum-like class
(PaletteItem) to reduce hidden dependencies of the palette item names.

Moreover, ConsoleWidget is now a urwid.Pile with Text widgets, instead
of one single Text widget with all text. This also allows for doing
operations on lines in the output in the future (mark a line and hit
complete, for instance).

PrettyPrinter no longer outputs a string, but a TopydoString in case of a
single todo item, or a list of TopydoStrings that may contain color
information. Those who read the PrettyPrinter output should convert it to a
string that can be displayed.

TopydoString is a string descendent that maintains information where
colors should be applied in a string. A user interface can pass a
function that transforms a color to a color representation that is
relevant there. That way, the topydo library does not make any
assumptions anymore on where colors should be applied.

Improve consistency of help messages.

Allow using `tag -a` multiple times

Use special color-palette entry for 'marked' items