Built-in shortcut for loading into process.env

[sholladay]

The recipe for loading the environment variables into process.env is pretty simple, but it's nonetheless more cumbersome than it ought to be. We should include this feature as an optional part of our API.

My thought is something like:

envy.assign('.env', process.env);

Or, given that #5 will be implemented, perhaps:

envy.assign({ target : process.env });

Not sure about the names assign and target here. Suggestions for other names welcome.