Allow extensibility via custom features

[kstevens715]

If there were a way to add entries to PgSearch::ScopeOptions::FEATURE_CLASSES, then users could extend pg_search with their own custom features. For example, I created a Trigram (Feature) sub-class that adds some additional scoping which greatly improved the performance of the resulting queries.

I should also mention #542 would also probably allow me to solve the same problem. However, I think there's value in having both options available.

pg_search/lib/pg_search/scope_options.rb

Lines 136 to 140 in 08df66a

	FEATURE_CLASSES = { # standard:disable Lint/UselessConstantScoping
	dmetaphone: Features::DMetaphone,
	tsearch: Features::TSearch,
	trigram: Features::Trigram
	}.freeze

Right now, I'm getting around the fact that FEATURE_CLASSES is frozen by doing this:

Rails.application.config.after_initialize do
  feature_classes = PgSearch::ScopeOptions::FEATURE_CLASSES.dup.merge(
    scoped_trigram: PgSearch::Features::ScopedTrigram,
  ).freeze

  PgSearch::ScopeOptions.send(:remove_const, :FEATURE_CLASSES)
  PgSearch::ScopeOptions.const_set(:FEATURE_CLASSES, feature_classes)
end

Ideally, I could add a new feature like this:

PgSearch.add_feature(scoped_trigram: ScopedTrigram)

If there is interest in this feature, I would be happy to make a pull request with tests. Please let me know!