Add env var prefix support

[hltbra]

This pull request needs some work if #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?

[heynemann]

I like this a lot. Will give it some thought and review soon. What do you think @guilhermef ?

[guilhermef]

I like this too, we never had this problem since we run most of our apps on isolated machines.

[rafaelcaricio]

What is necessary to make this move forward? I would like to use this feature in spec-first/connexion#367. Since it is a framework and naturally will be mixed with people's code makes sense to prefix all environment variables.

/ping @hltbra @heynemann @guilhermef

[heynemann]

Well, first rebase with master then make all tests pass and add a couple more tests if required.