Add config validator and test
Move config validator and test to core namespace

No other changes in this commit.
Update CHANGELOG

No need to bump version, as will do the config loader as well.
Merge branch 'master' into add-config-validator

Conflicts:
	CHANGELOG.md
Config validator test should be in core too
Merge pull request #11 from alphagov/add-config-validator

[#65625634] Add config validator