preserve db-specific options in schema.rb, fixes #148

basically any valid db-specific thing that you can already specify via
`options: "..."` will be preserved on the way back out.

for mysql that's:
* `ON UPDATE <action>`

for postgres that's:
* `ON UPDATE <action>`
* `ON DELETE SET DEFAULT` (doesn't map to a `:dependent` value)
* `DEFERRABLE` (`NOT DEFERRABLE` is the default)
* `INITIALLY DEFERRED` (`INITIALLY IMMEDIATE` is the default)
* `NOT VALID`

postgres' `MATCH <type>` is not supported, but that's already problematic
for `:options` since it is incompatible with `:dependent` (due to where it
appears in the statement), so ¯\_(ツ)_/¯