feat: support for `outFile` and `sourceMap` node-sass config options

[marksmccann]

• outFile supports the same features as output but will not write to disk.
• If a function is passed to sourceMap, it will execute on the output path and use the returned path.
• sourceMaps will be written to disk when provided along with output.
• Logic used to determine output was moved out of render and into a stand-alone utility.
• Contatenation of multiple sources to a single output was removed temporarily due to incompatibility with source maps. I created #19 for us to add this back later.
• Plus minor updates to utility functions and comments.

closes #18

[marksmccann]

@phillipluther This PR is ready for review. Just a heads up, I attempted to add the concat features with the strategy we discussed but ran into some road blocks. I decided to just temporarily remove the concat logic entirely since it was not compatible with source maps. I created #19 for us to pick up later to re-add. I figured it was better for me to move on than to get held up.

Also, removing that logic was technically a "breaking change" but since we haven't released the package yet, I didn't label it as one in the commit. Was that the right call?

[phillipluther]

@marksmccann Awesome! Taking a look now.

I'm at peace with removing the concatenation behavior. Inclusions of it was all based on theory/neatness; it's not functionality that node-sass provides and we have a perfectly acceptable -- I'd even argue standard -- work around by just making manifest files that @import the sheets you want to concat and passing paths to those. We can revisit once we get some usage feedback.

RE: breaking change, totally on board. It's breaking for whom? Me? You? I don't think we have to sweat breaking changes since those versions will never show up in NPM and it'd be impossible to experience a situation where going from one pre-release version to another breaks anything.

That is, one would never be able to npm install node-sass-extra@0.10 and then have npm install node-sass-extra@0.1.1 (or whatever) come in a break it. The module history starts at publish. I think that was the right call, in addition to not version bumping it at all.

[phillipluther]

Everything looks great! Informal approval.