Making proper_table_name take in options.

The options will specify the prefix and the suffix. Also, I'm moving the
method to be an instance method on the +Migration+ instance. This makes more
sense than being a class method on the +Migrator+ class because the only
place that uses it is on a +Migration+ instance (in a method_missing
hook). The logic for the Migrator shouldn't be doing any work to
calculate the table name, it should be the Migration itself.

Also made some small indentation fixes.
Merge pull request #11984 from wangjohn/proper_table_name

Making proper_table_name take in options.