Add support for custom post-save actions

Sometimes a user will want to save and continue editing, or add another piece of data. Rather than redirect them to the index action and have them suffer through that process, we allow setting any of the following pieces of POST data:

- `_add`: Will save the record and redirect to add form. Redirection may not always be necessary.
- `_edit`: Will save the record and redirect to edit form. Redirection may not always be necessary.
- `_save`: Will save the record and redirect to index page.
Add buttons for custom post-save actions