Initialize config with commented out defaults.

This should fix #467, sample output:

```
# For further information about options in this file see:
#   http://docs.mopidy.com/en/latest/config/
#
# The initial commented out values reflect the defaults as of:
#   Mopidy 0.17.0a1
#   Mopidy-HTTP 0.17.0a1
#   Mopidy-Local 0.17.0a1
#   Mopidy-MPD 0.17.0a1
#   Mopidy-Spotify 1.0.1
#   Mopidy-Stream 0.17.0a1
#
# Available options and defaults might have changed since then,
# run `mopidy config` to see the current effective config.

[logging]
#console_format = %(levelname)-8s %(message)s
#debug_format = %(levelname)-8s %(asctime)s [%(process)d:%(threadName)s] %(name)s\n  %(message)s
#debug_file = mopidy.log
#config_file =

...
```