Support envrc

[glasserc]

Fixes #446.

I didn't see any tests for this code and wasn't exactly sure how to add one. However, I tested it by eval'ing the updated function in my emacs session and running rust-test. Before the change, it would error out, and after it successfully runs the tests.

It turns out setting process-environment wasn't enough for my situation so I set exec-path as well.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brotzeit (or someone else) soon.

Please see the contribution instructions for more information.

[brotzeit]

Thanks!