config: Return convereted values and errors
config: Rename convert to deserialize
config: Fix handling of None in Secret
config: Add core_defaults to config
main: Refactor to use underlying _load and _validate functions from config
ext: Remove filter_enabled_extensions
Merge branch 'develop' into feature/config-api-cleanup

Conflicts:
	mopidy/__main__.py
	mopidy/ext.py
ext: Switch to validate_extension(extension, config)
config: Remove ConfigErrors expception
config: Move all code to new load function
ext: Do not compound config enabled and validate_extension
config: Unknown loglevels should serialize to blank string
config: Remove format from types API
config: Fix serialization of none
config: Set missing/invalid keys to none
config: Create config.format and switch to just serialize on schemas
ext: Typo in docstring
config: flake8
Merge pull request #417 from adamcik/feature/config-api-cleanup

Move towards stable config APIs.