Add support for creating an index via migration generator

I added a migration generator to create indexes. To invoke, use the generator from the command line:

    rails g migration IndexAuthorNameAndTitleOnBooks author title

That will create a migration that adds an index named `index_author_name_and_title`
on the `books` table, using the `author` and `title` columns:

    add_index :books, [:author, :title], name: 'index_author_name_and_title'


Thoughts?