Allow environment variables to be set for the sass process.

Environment variables are a common way for Sass extensions to be configured. For instance, at my work we set a LOCALE environment variable to create for per-locale output.

This allows an environment hash to be passed to the sass processor and then passes it through when the sass process is spawned.

My test fails but it's failing the same way that a bunch of the other tests are failing. I'm not sure why, but I think the test will pass if that is sorted out.