Refactor configuration to support easier extensions

[furious-luke]

It's currently a bit ugly to add more configuration options. I'd like to make it more declarative. There are a few steps.

• [x] Update OptionDict to be a bit cleaner.
• [x] Make a declarative version of OptionDict that supports merging.
• [x] Allow options in the option dict to extract their values from the environment.
• [x] Remove configuration from application context and use a proxy instead.
• [x] Create a configuration class decorator that supports mounting configurations to the existing configuration.
• [x] Support environmental defaults from nested configurations.

[furious-luke]

Am putting this on the backburner for now. Have implemented a little bit of it, but I need to think some more about the right way to structure the meta and the resolvers.