Support nested config structures too

[cknv]

Sometimes you need dictionaries to configure things. I have encountered this most often with 3rd party tools that you integrate into an app.

This will make envargs able to support nesting vars gracefully such as:

settings = {
    'nested': {
        'MY_VAR': Var(use=str),
    },
}

And envargs will recurse properly to take care of the nested Vars.

I suppose this also makes it trivial to add static config vars later on, but that is beyond this PR.

[coveralls]

Coverage remained the same at 100.0% when pulling 51e4f12 on nesting into 26df9cc on master.