add sourcemap option set up logic

Handles setting up smarter defaults for the sourcemap property.

- sourcemap property is still wonky (as of testing on 0.8.1) as it
  only uses the filename and disregards path.

Most of this logic is from:
https://github.com/andrew/node-sass/blob/9b752b2729f039590f15ffe239a61e60d0984c77/lib/cli.js#L119-L132

Resolves #57 and #58