fixes #71

BREAKING CHANGE: `MetaElement.attribute` has changed from `string[]` to
`MetaAttribute[]` and both `requiredAttributes` and `deprecatedAttributes` has
been merged into object. This only affects API usage, primarily custom rules
accessing attribute metadata.See `MIGRATION.md` for details.

The old format is deprecated but is internally converted as to not break user
configuration. For compatibility it will probably be around for quite a while
but users should try to migrate as soon as possible.