Closes#229
When there's no host given in a postgres connection url, use the unix
domain default socket. Previously, the default was a localhost TCP
connection. This default behavior is more in line with standard postgres
clients out there, like psql.
`dbmate` package was starting to get a bit polluted. This PR migrates each driver into a separate package, with clean separation between each.
In addition:
* Drivers are now initialized with a URL, avoiding the need to pass `*url.URL` to every method
* Sqlite supports a cleaner syntax for relative paths
* Driver tests now load their test URL from environment variables
Public API of `dbmate` package has not changed (no changes to `main` package).
