Split the CLI class into a CLI base class.

Implementation of editing todos in $EDITOR

implements #2

- `topydo edit` will still open todofile in $EDITOR
- `topydo edit 1 2 3` will open tmpfile with corresponding todos
- `topydo edit -e @Home due:today` will open tmpfile with todos matching
   the expression

This is done by applying the colors as the last step, so the preceding
filters don't have to worry about it.

Introduce MultiCommand class for cmds operating on multiple todos.

Commands operating on multiple todos will inherit common execute()
method from MultiCommand class, and part specific for each command will
be covered in execute_multi_specific()

It will be common for all commands operating with multiple todo IDs.

topydo edit won't work if EDITOR is unset

On some systems EDITOR is unset by default, so in such situation topydo
would throw KeyError exception instead of opening todo.txt in vi.

More output and behavior consistency between 'del', 'do', 'pri and 'postpone'

Append parent contexts

This works identically to append_parent_projects option. Code was
generated by simply `s/project/context/`

Adjust to changes introduced in: 3f096

1. Fail if *any* of supplied todo numbers is invalid
2. Print which todo numbers were invalid

pri:
    + We're now printing todo numbers in output
    + Test error output when any of supplied (multiple) todo IDs is invalid.
both:
    + Test error output when *all* of supplied todo IDs are invalid

Show 'Invalid todo number: {number}.' messages also when *all* of
supplied todo numbers are invalid. Before this commit it were returning
synopsis in such situation.

Give 'postpone' command possibility to work with multiple todos

topydo will report which todo IDs ('numbers') were invalid if user
supplied more than one on command-line. Error for operation on single
todo ID are reported as usual (for consistency with other commands sake
and because I'm lazy and don't want to fix 2 tests :P).

Allow multiple tasks to be prioritized at once

1. Shorter code for creating and populating 'todos' list
2. Merge IndexError and InvalidCommandArgument

@bram85: Thanks!

Fixes issue #5

When there is a todo in the list with an id: tag but no child todos,
that value could be reused when creating a new dependency between two
(unrelated) items.

This is because the value of the dangling parent todo is not present in
the dependency graph, but the function generating a new ID depended on
that.

Instead, simply iterate over all available todo items to check whether
the proposed value already exists somewhere.