Lookup mechanism:

1. Check whether an editor was given with the -E flag:

     topydo edit -E vim

2. Use the value of $TOPYDO_EDITOR
3. Use the value in the configuration file:

   [edit]
   editor = vim

4. Use the value in $EDITOR
5. Use 'vi' if the above fails.

This makes it easier to invoke an editor as vim with additional
parameters, for instance by adding the full todo.txt file to the
completion options (see the tip in issue #164). This trick was added to
the configuration file as a comment.

Suppose we have the todo item:

    (C) Some item key:value1 key:value2

And then I do an ls:

    topydo ls -x key:value2

Before, this would yield an empty result, which is undesired. Therefore,
only apply ordinal tag filtering when a tag appears exactly once in the
todo item. This is a simple approach, and special operators >, >= etc
will not be applicable:

    topydo ls -x key:>value1

will no longer work. The result could be ambiguous: filter out when none of
the tags match, or only some of them?

Don't crash on FileNotFoundError with `add -f`

Show descriptive error message instead.

Relative dates were already interpreted for due and start tags, but you
can also enforce it for other tags by passing the -r flag to the tag
command.

* Extended the feature list.
* List the UIs.
* Added a screenshot of the column UI.
* Dropped information on dependencies, the user should not worry about
  this with pip. Or just look at setup.py.

Todo items with " & < or > were not properly rendered because they were
not escaped.

Thanks to Jacob Leemaster for the bug report.

Normally, pressing ':' after executing a command would hide the console
(equivalent behavior to Escape or Enter). Now, : will focus the
commandline while keeping the previous output visible. This makes it
easier to compose a new command based on a todo shown in the previous
output (for example perform an 'ls' and then a 'dep' with IDs
afterwards).

Escape and Enter still have the same behavior, which can be used to
dismiss the last output.

When no valid grouping could be made, an empty header would be printed
(0 length group label, zero = characters below).

The empty tuple represents the initial grouping.

Fixes #153

This reverts the first attempt (commit
8310b029), because relative dates in terms of
hours were printed.

This attempt passes the now object that we just created to humanize(), so that
will be used as a offset. If we don't supply that object, arrow will create a
different datetime object representing 'now', which might be slightly off
w.r.t. the now object we created.  Therefore the floating point arithmetic done
within the arrow library might not produce the desired results (delta / 86400).

By passing the offset ourselves I was able to produce reliable
results.

With the previous method of using arrow, the same date object would
result in different humanized dates. Apparently using now() and then
replacing the date components triggers some inaccuracy.

Simplify the call to arrow by passing it the datetime object directly.

It will simply gather all todo items in all groups and pass them to
print_list

When grouping by project, todo items without a project wouldn't be
listed (same for contexts).

Moreover, sort the projects such that the items without project are
shown at the bottom of the list.