Fixes #20.

Tags with a value of length 1 were not highlighted with the metadata color.

Compare:

http://pythex.org/?regex=\b\S%2B%3A[^%2F\s]\S%2B\b&test_string=star%3A1%0Astar%3A12&ignorecase=0&multiline=0&dotall=0&verbose=0

http://pythex.org/?regex=\b\S%2B%3A[^%2F\s]\S*\b&test_string=star%3A1%0Astar%3A12&ignorecase=0&multiline=0&dotall=0&verbose=0

If there was a todo with 'foo:somestring', it wouldn't be returned with
the command above, because somestring couldn't be converted to a date or
an integer, and therefore the OrdinalTagFilter just returns False.

As a final fallback, use a GrepFilter match to do the string comparison
such that the todo item(s) with this key:value combination will be
returned.

Priority Aa or AB is (still) not allowed.

Conflicts:
	README.md

The previous commit had the archive condition on the wrong place,
resulting that todo.txt files were not written out at all if archiving
was disabled.

Also introduce the (overall) -a flag to disable auto archiving. -a comes
from the original todo.txt CLI.

It would have been nicer to have DoCommand support the -a flag, however,
DoCommand doesn't know anything about archiving and I would like to keep
it that way.

Addresses issue #42.

(Note that commenting/omitting archive_filename will still by default
write the archived entries to done.txt).

This addresses issue #42.

Conflicts:
	topydo/lib/DCommand.py

Expression-mode for del, depri, do, pri, postpone.

Fell into the trap of calling a function as a default parameter. Because
of this, relative dates used with the 'add' or 'do' command (recurrence)
were based on an offset when topydo was started, not when the subcommand
was executed. Usually these two events occur in the same second and this
bug was unnoticed for a long time. But now with prompt mode, this bug
may surface if you leave running topydo for longer than a day.

- add test for direct todo.txt editing
- remove unneceseary `u()`

- execute_multi_specific() -> _execute_multi_specific()
- execute_not_multi() -> _execute_not_multi()

MultiCommand._catch_todo_errors() prevents occuring of those.

This required some refactoring in MultiCommand and ExpressionCommand.

Whole part about processing flags was refactored. Code related to flags
from DCommand was moved into MultiCommand and reused from there in all
"multi" commands.

New attribute: ExpressionCommand.last_argument. Should be set to True if
command is using the last argument as something other than todo ID or
part of an expression (as in case of postpone or pri). By default set to
False.

New method: MultiCommand.execute_not_multi(): wrapper for functionality
not connected with todo IDs (like `topydo edit` or `topydo edit -d`).

New attribute: MultiCommand.multi_mode: Should be set to True if user
wants to operate on multiple todo items. True by default.

New method: MultiCommand.get_todos_from_expr(): gets todo IDs based on
expression and puts it in MultiCommand.todos.

EditCommand._open_in_editor() now accepts only one argument - file to
editing. It handles now also OSError exceptions. It is used also to
opening todo.txt and done.txt.

MultiCommand.get_todos() doesn't accept anymore todo IDs as an argument.
Instead it's using inherited Expression.last_argument` to decide upon
where to find todo IDs.

Now desired result of DCommand.get_flags() should be more apparent.

Now every call to editor is done by `EditCommand._open_in_editor()` and
whole process of getting editor name is also done there.

When a subcommand finishes, _post_excecute() is called for sorting,
archiving and writing the file. But sorting is also a subcommand, which
in turn also calls _post_execute(), etc, etc.

So let the caller of _execute() decide when to call _post_execute.

This fixes issue #46.

* Make TodoListTester test TodoListBase, make
  TodoListCleanDependencyTester test TodoList. This required moving a
  test case from the former class to the latter.
* Add an additional test for a non-existent todo.

Just make sure that proper arguments are passed to GrepFilter in the
first place.