Add `add_reference/remove_reference` (safe version)

[tragiclifestories]

This issue is designed to track the 'safe' version of implementing add_reference.

Rails, by default, indexes reference columns. This is usually a good idea (and can be disabled by passing index: false when it isn't). However, it adds indexes in a naive, unsafe way, without using CONCURRENTLY to maintain availability.

To fulfil this ticket, we must:

• [ ] Re-add add_reference and remove_reference, setting index: false in the output[1]. If index: true is explicitly set, a validation error should explain that this is not allowed. (#82)
• [ ] Add a generator for add_reference, that will generate two migrations - one to add the column (or columns, in the case of a polymorphic key) and one to add an index.
• [ ] Change generator for foreign_key, changing the first migration to use (add|remove)_reference and adding a fourth to create an index on the column. There should be a --no-index class option that disables the index creation.

[1]: Weirdly, Rails seems to ignore the index option in remove_reference, despite using it in one of the examples. This needs to be investigated further.