Add optional escaping to interpolated values

[bobjflong]

Please feel free to push me in the direction of something more idiomatic if this isn't working, I'm pretty new to Elixir.

I have a need for auto escaping of interpolated values. As mentioned by https://github.com/nurugger07/calliope/issues/24. In my opinion one advantage of templating libraries is taking care of those kinds of problems. It might even be a good idea for this to be a default 💭

Thanks!