nimrod-lang: New and improved formulas

This pull request attempts to improve Homebrew's support for the Nimrod language by supplying two new and improved formulas. The explanation for the changes are in the commits themselves, and I'd also be happy to answer any questions about this here.
Needs a `url "https://github.com/nim-lang/nimble/archive/v0.4.tar.gz"` line for a stable build.