Remove dependency on all drivers

[jbuchbinder]

The current setup loads all drivers, rather than having them loaded by the end user. This means that any dependency which requires CGO to build -- and is therefore not able to be cross-compiled. This seems to be the case for at least the PostgreSQL and Sqlite3 drivers.

I'm seeing this paradigm in the driver code : https://github.com/mattes/migrate/blob/master/driver/driver.go

Instead of calling a single driver.New() method, it might make more sense to have a New() method implemented by each driver?

[AlekSi]

+1. Also it would be nice to have build tags for specific drivers.

[danilobuerger]

👍

[mattes]

Please see #54