Add require_environment setting.

This branch adds a single new config setting to force the environment file to be present and readable. If set to true, Dancer will fail during startup if your environment file is missing or can't be read. This is especially useful in production when you have things like memcached settings that need to be set per-environment. It also helps catch typos in the environment file, and helps avoid production from running with the default 'development' environment settings.  I thought about building this as a plugin, but it's actually a very small amount of code and I think it makes more sense as part of the core.

I've tried to match style and naming conventions, but please let me know if you want it cleaned up at all.