Enhance Suffix#config so it can be usefully overridden by downstream

apps by:

- pushing the list of available suffixes and their order into the config
- push type validation into the type_suffix proc
- expose the config in a class-level method

I think this makes it possible to do a reasonable Solrizer 2.x-compatible suffix config, for those of us unfortunate enough to be stuck with the previous schema for some time to come.

This should be a minor-release compatible change, only the private config API has changed.