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
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.
local: Add file ext blacklist (fixes #516).

Adds a new local/scan_blacklist_extensions config value for controlling file
extensions to ignore.
Merge pull request #549 from adamcik/fix/bug-500-library-refresh-does-not-remove-uris

local: Delete uris in library refresh (fixes #500)
Merge pull request #548 from adamcik/feature/write-config

Prep work for adding a `mopidy config --set ...` type command.
Merge branch 'develop' into feature/track_and_count
mpd: Fix import order
mpd: Formatting
mpd: Add 'find track ""' test to match search tests
local: Bring the corner case close to the condition
local: Order search filters consistently
local: Readd lost test assertion