Adding in extension support

[jwoertink]

This PR will be needed to do #122 and #188

This adds in helper methods for enabling and disabling extensions.

enable_extension "uuid-ossp"
disable_extension "uuid-ossp"
update_extension "hstore", to: "2.0"

[paulcsmith]

Looks good man! Is it ever possible/necessary to pass args when enabling extensions? Like extension options or anything?

Im not sure so if not that’s fine!

[jwoertink]

There's a few small options you could technically use like specifying the schema if it's not public, or installing an extension from a specific older version, but those seem to be super edge case since we currently don't support schemas outside of public, and I think you can just use the update_extension and specify a different version if you need to roll it back.

[paulcsmith]

This all looks great! Could you add an issue to website so we remember to document it?