Add precompile command

I've added a console command to precompile twig templates, which is handy when you're doing a deployment.

What it does is it goes through your `$laravel['path']` and will load each `*.{twigbridge::extension}` template. This has the pleasant side-effect of creating the cache for each file.

I'm relatively new to the Laravel world, so feel free to make any changes if they're not particularly idiomatic. I'm especially iffy about the tests I've written - there's a *lot* of mocking code in there.