Extra field def options for Solr, and custom field analyzers

Based on https://github.com/silverstripe-labs/silverstripe-fulltextsearch/pull/4 and https://github.com/silverstripe-labs/silverstripe-fulltextsearch/pull/6, can't separate because of the SolrIndexTest base class.

I didn't actually end up using the custom field analyzers, since I've later discovered that the 'htmltext' type already stripped HTML tags, and it was simply detecting the wrong field type in addFilterField(). Still a useful addition though.