adds nested config support via env variables + tests

tried to keep the code as stylistically close as possible

##### nested config from `"__"`-delimited env vars
https://github.com/mikermcneil/rc/compare/dominictarr:master...master#diff-50e3aa130a4f97a42ee2cf111c7b1d9dR36

##### tests
https://github.com/mikermcneil/rc/compare/dominictarr:master...master#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R8