Properly unmark items when resetting state

After introducing cache for TodoWidget objects, widgets highlighted with
'mark' action were not unmarked after 'reset' action and they reappeared
highlighted ("marked") after reverting `do` or `delete` commands.

This serves two purposes:

* Relative dates and progress colors are shown correctly on the next day
* Stale cached items get wiped at least once a day (for those who continiously
  run this mode)

When editing one or more todo items, some items may not have changed, therefore
having the exact same source text. The EditCommand still deletes the Todo
instance and creates a new one and puts it in the todo list. But the widget
cache only looked at the source text, found a widget from before the edit and
will use it. But the widget still has a reference to the old Todo instance.
When executing an action on it (e.g. mark it as complete), the TodoList doesn't
remember that todo item, which results in a crash.

Fixed by updating the reference to the latest Todo instance when the sources
are equal but the instances are not.

Having hundreds of todo items slows down the column UI quite a lot when doing
an update. An update basically throws away all widgets and creates them from
scratch. That is quite a waste when only one item has changed.

Instead, keep all widgets in a cache, retrievable by the todo's raw text. When
the raw text changes, a new widget is constructed.

When topydo is run with colors, items without priority get an invalid
markup added. Only add markup when an item has actually a priority.

Bug found and fixed by @mruwek

Fix saving backups when archiving is turned off

The parent color is only used when the todo item has not enough information to
determine its own progress.

Use empty list if user left `archive_file` option empty.

This fixes #132

Allow `--version` as an alias for `-v`

Provides a smart way to determine the number 'top' lines

Add a long version option

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