Add template encoding option

[timorieber]

Fixes #445.

I have implemented a new option to specify the actual template encoding and extended the loader mechanism. Also did some refactoring in the read_templates function for better readability and testability.

[rmilecki]

I'm not sure if we need all that encoding at all. We should also clarify that unclear

# -*- coding: utf-8 -*-

like syntax.

As for you actual changes here, your commit "Implement yaml option for template encoding" does a lot of refactoring and rewording that isn't related to actual issue. In my opinion that should go to another pull request so we can focus on one thing at a time.

[timorieber]

This PR is meant as a short-term backwards-compatible approach to this template encoding issue. For the longer term I support your plan of removing this "magic" and assume utf-8 template encoding.

I had a harder time than expected understanding what's going on in this code. As I'm a big fan of Robert C. Martin's "boyscout rule" to "leave the code better than you found it" and Martin Fowler's "opportunistic refactoring" I first wrote some tests (there were none) and afterwards split things up a little bit. I'm pretty confident that it works just like expected.

I'm also fine when this PR gets closed, but I think it's the right direction.

[rmilecki]

Obsoleted by #449

Cleanup of existing templates is pending: #452