Merge pull request #380 from adamcik/feature/config-optional

config: Add optional setting to config values and improve tests.
spotify: Add config schema
local: Add default config and config schema
stream: Add default config and config schema
mpd: Add default config and config schema
lastfm: Add default config and config schema
mpris: Add default config and config schema
http: Add default config and config schema
docs: Tweak module docs formatting
docs: Update extensiondev docs to match real usage
local: Return schema
Merge pull request #381 from jodal/feature/add-ext-config

Add default config and config schemas to extensions
docs: Update extensiondev implementation plan
main: Load extensions earlier
config: Add default core config
ext: Replace validate_config() with get_config_schema()
docs: Tweak extensiondev docs
ext: Add both dist_name and ext_name to Extension class
ext: Change entry point from 'mopidy.extension' to 'mopidy.ext' to be more consistent
main: Collect and validate config from all sources