Collect config from all sources and validate it

Step 9 from http://docs.mopidy.com/en/develop/extensiondev/#implementation-steps

> Switch to ini file based configuration, using ConfigParser. The default config is the combination of a core config file plus the config from each installed extension. To find the effective config for the system, the following config sources are added together, with the later ones overriding the earlier ones:
> - the default config built from Mopidy core and all installed extensions,
> - /etc/mopidy/mopidy.conf,
> - ~/.config/mopidy/mopidy.conf,
> - any config file provided via command line arguments, and
> - any config values provided via command line arguments.

This isn't ready for merge, but I want to share the current state, which is actually working.