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.
Merge pull request #548 from adamcik/feature/write-config

Prep work for adding a `mopidy config --set ...` type command.