Add support for 'count ..' and 'find .. track X'
Error checking before type-cast
Changed track to track_no, fix code, add tests

Added testcases for track and count
Return MpdArgError if count parsing fails
Fix tabs->spaces and extra ')'
Fix small bug and fix test
Remove duplicate test group, merge it into existing
docs: Enable new RTD theme
docs: More exact GStreamer version range
docs: Update authors
docs: Fix link to Mopidy-Arcam
docs: Fix typo
docs: Fix typo
docs: Search is on every page, don't need a search page
mpd: Fix flipped mute logic
local: Fix crash on non-ASCII chars in URIs
docs: Warn about EOT issue with Shoutcast (fixes #545)
config: Add preprocessor for preserving comments when editing configs.

Adds markers to configs files that ensures configparser won't mangle comments
in the files. Will be combined with a postprocessor that undoes these changes.
config: Add postprocessor for converting config back.

Idea forward from here is that once we have a config sub command that we expose
a setting config values which will:

1. Run the preprocessor on the file to edit.
2. Load it into config parser.
3. Modify the value.
4. Write it to a io.ByteString
5. Run the postprocessor
6. Save the file with comments etc intact.
local: Delete uris in library refresh (fixes #500)

Makes sure we remove uri's that can no longer be found in the tag cache.