#245 - Initial support for expression-based inclusions

Inclusion now supports expression-based template resolving, by pushing the parsing and rendering of the target template into the render step of the include itself. It's a little dirty, but it works.

Of note is that we ensure that if the template is specified as a constant, that we determine that it exists, or handle the 'missing' component such that an exception is returned as early as possible, in order to conform to the earlier acceptance tests. Thus, this is a backward-compatible change, and may possibly be scheduled for release in 4.0.0