Add support to output enum definitions in the db doc README

[insano10]

I'd like enum support for the documentation. It's not finished yet as there's no tests, but can someone give me feedback if I'm going down the right lines? I'm only using Postgres at the moment which is why it's only been implemented for that driver.

When I build and run tbls against my local db, the enum output looks as expected in the README.md.