python: Support multiple grabber modules for movies and tv artwork.

python: Support multiple grabber modules for movies and tv artwork.

The "--modules-movie=a,b,c" command line option will load each of
these modules in turn and ask them to provide artwork. TV modules are
selected via "--modules-tv" option.

The default (if no --modules-movies is provided) is to search the
Python path for python files that are named "tv_meta_*py" and then
call the module's "get_capabilities" to determine if the grabber
supports tv, movie, or both.

Each grabber module is called in turn until all artwork is
retrieved. So, if the first module only provides a fanart, then the
next module can supply the poster image.

Modules can be passed command line options from tvhmeta by prefixing
them with the shortened module name.

So, for a grabber module such as "tv_grab_tmdb", we pass through
command line arguments from tvhmeta to it. So if the tvhmeta is called
with an option of "--tmdb-key" then we pass through "key" as an option
to the tv_grab_tmdb (stripping the "--tmdb").

This allows third parties to produce grabber modules that
automatically integrate.

Also switched from deprecated OptionParser to argparse.