Create sourcemaps with gulp-sourcemaps

Nerfed the sass options a little here for good integration with gulp-sourcemaps.
We don't support auto or inline values for sourcemaps since they give us nothing
extra when using gulp sourcemaps. Opinions over choices.

This is the only part of the plugin that does something a little special, beyond
just spawning sass, so is more prone to errors that are our fault.
Ignore test artifacts
Add basic tests for inline and file sourcemaps

Still to come: tests for source file paths.