Automatically add loadpaths for intermediate files

Work in progress, but good enough for some testing. Usage feedback welcome.

To test, add

```
"gulp-ruby-sass": "git://github.com/sindresorhus/gulp-ruby-sass#rw/auto-loadpath"
```

to your package.json and then run `npm install`. 

After you do this, try running the plugin with any single or multiple file glob, only adding the relative loadpaths you would use if you were running Sass directly — pretty much the same configuration you would have used for 0.5.0.

Sourcemaps seem to work better too.