Add check for invalid keys in config files

[alainmeier]

It is currently possible to include invalid linter names in the config file. If an invalid name is included, an error should be thrown rather than silently ignoring it. (As seen here #31)

[alainmeier]

It appears there is some kind of handling for invalid linter names, but the config keys don't get far enough to trigger the err https://github.com/sds/slim-lint/blob/master/lib/slim_lint/linter_registry.rb#L31