Creating migrations from a template

[jprokay]

👋 I am a big fan of the simplicity of dbmate, but I do miss one feature from Sqitch: Templates
It is incredibly convenient to have templates, especially when working with a team, to ensure uniformity of migrations

I created a PR to add support for templates
I haven't tested the crazy use cases that are possible with the text/template library, but for my use cases, it works great.

[dossy]

@jprokay Do you want to submit the pull request to the amacneil:main branch?

[jprokay]

Yeah, I will do that. Thanks!

[dossy]

#530