add sourcemap option set up logic

Handles setting up smarter defaults for the sourcemap property.

Most of this logic is from:
https://github.com/andrew/node-sass/blob/9b752b2729f039590f15ffe239a61e60d0984c77/lib/cli.js#L119-L132

Resolves #57 and #58

Note: I didn't add tests for this change. Also, I added a FIXME note because it's getting late and I haven't thought about how whether to use the cwd.