Sort indexes alphabetically, like Rails does.

Rails sorts indexes alphabetically when dumping a schema;[<sup>[1]</sup>](https://github.com/rails/rails/blob/be71b0ecbe4b6f2416296dd0c120b652151ebcaa/activerecord/lib/active_record/schema_dumper.rb#L191) it has done this since 2008.[<sup>[2]</sup>](https://rails.lighthouseapp.com/projects/8994/tickets/1266-order-add_index-statements-in-schemarb)

The members of our team have indexes in different orders, depending on when they created their databases by loading the schema and how many migrations they've run chronologically since then.  Thus, when we started using schema_plus, we all started dumping our schemas with different index orders, which led to a lot of spurious git diffs.

I considered just making a migration to change the ordering of the indexes for everyone.  But, I figured it would be better to make schema_plus behave the same way that Rails has since 2008.  Feel free to close this pull request if you disagree!