Much simpler runtime-only inclusion

@kusmierz asked me if we could simplify the runtime-only inclusion to be able to run a project without adding a lot of hard to catch configuration, so I dig in and found that we could simply add a plugin configuration value to specify where the handlebars compiler is.

So here it is. This version is not retrocompatible with my previous approach, but is much more simple to understand. I'm sorry I didn't think about this the first time!

Feel free to take a look and ask me for clarifications if you need it.