Initial import of missing tracks plugin

This plugin adds a new command, ``missing`` or ``miss``, which finds
and lists, for every album in your collection, which tracks are
missing.
Add documentation for missing tracks plugin

Also add link to the documentation plugin to the index
Remove unnecessary _format function
Remove unneeded Library argument from _missing
Add _missing_count function
Add 'count' and 'total' parameters

- count: count missing tracks per album
- total: count total missing tracks
Implement logic for displaying missing track counts
Don't check for item.paths == None as that can't happen
Synchronize documentation with new parameters and respective behavior
Modify "not part of beets" to "part of beets"
Merge branch 'missing' of https://github.com/pedros/beets

Conflicts:
	docs/plugins/index.rst