Add env var prefix support

This pull request needs some work if https://github.com/globocom/derpconf/pull/8 gets merged.

I came up with this feature idea because I would like to add env var support to https://github.com/thumbor/thumbor, and thumbor has some env var names that may conflict with other programs if env vars are shared (i.e `SENTRY_DSN_URL`).

What do you guys think?