Load ~/.powrc and ~/.powenv too

I'm not sure what you mean here, I can totally update the PR and `cd #{@root}` if you want.

I quoted because I was it's used [all over the place][quote] in `sourceScriptEnv`, the alternative I see is to allow setting cwd as an option.

[quote]: https://github.com/basecamp/pow/blob/master/src/util.coffee#L110-L141