Support include paths

[voxpelli]

When using include paths in combination with this importer the include paths are simply ignored.

When looking into how include paths could be supported I ran into the same issue as mentioned in #13: That one then would also have to add logic that looks for all valid variations of a Sass file to be able to step through each and every include path as well as the local path to figure out if a file is available or not.

Solving this would involve looking for these file names:

• name.scss
• name.sass
• _name.scss
• _name.sass

And doing so first locally and then on each and every include path until one either finds a file or finds no file.

Reference in Sass docs: http://sass-lang.com/documentation/file.SASS_REFERENCE.html#import