Adds support for templates when creating a new migration

[jprokay]

Inspired by Sqitch Templates

Concerns:
I set the default directory to ~/.config/dbmate/templates similar to sqitch. Should we consider different default paths based on the OS?
There are potential issues with needing to escape characters passed in via the CLI as mentioned in the #439

[amacneil]

Have you found storing templates in your home folder useful? I would expect something like that to live in your repo.

[jprokay]

@amacneil that's a great question. I do think most of my templates end up in the repo eventually. It depends on if I am working with others. The home folder can be convenient as a generic place to hold templates that I would use across different projects.