Split unsupported Regexp into an optional module

At this point tests are passing in Ruby 1.8.7!
One step towards multiple supporting multiple extensions
Allow for more Environment#load extensions in the future
Move variable loading to an extension
Clean up extensions, avoid explosing an extension API for now

I think it would be really interesting eventually to expose this to
other gems, but I’d like to experiment with it internally first.
Merge pull request #77 from bkeepers/extensions

Move variable substitution to an extension