Make migrate plugin based

[JensRantil]

Currently all database backends must be compiled into migrate. Some issues with this:

• There is a short-term annoyance with this that a recompilation is needed to add a new backend.
• There is also a long-term risk that two backends will require different incompatible versions of a third party library.
• Will allow external plugins outside of this repository to make each plugin maintained by different maintainers.

What are your thoughts on this?