git-cola: allow setting the filter text from the command line

We can now say:

	$ git cola --status-filter .
or
	$ git cola -s .

... and the status filter will be preset with the appropriate filter.

Related-to: #337
Suggested-by: @hrj
Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola: add a --help-commands option

We previously special-cased '-h' and '--help' in sys.argv so that we
could print the available sub-commands help message when running
`git cola --help`.

It makes more sense to print the cola command's help message since it is
the default command that is when no sub-command is specified.

Add a separate '--help-commands' option that can be used to print the
available sub-commands.

Signed-off-by: David Aguilar <davvid@gmail.com>