Allow access to native template features

[philwhineray]

At present the Handlebars and Tera template functionality is fully encapsulated.

It is likely that users will choose one based on features unique to that engine. Rocket should be able to expose the appropriate objects to make those features available.

See PR #60 for initial discussion.

[kardeiz]

I'm feeling this limitation too.

I think an acceptable resolution would be to just make rocket_contrib::templates::Context public. Then users could write their own fairing, which could modify e.g. Tera through context.engines.tera after calling Context::initialize, and then add the Context to Rocket state.

Context doesn't seem to contain any information that must be kept private. It's not ideal to make users set the templates directory manually (as in my scenario above), but is not that difficult, and that part of Template::fairing could be abstracted out into a Template::root function.

[seanlinsley]

I've submitted a new PR if anyone wants to try it out: #500.