Sass files getting processed by Tilt

[hkrutzer]

This causes source mapping using external files to fail, because these files cannot be rendered separately nor should they be (the original source should be returned).

Binary files are passed through the renderer as well (unsure if this is a problem), e.g.:

00:59:10 roger.1 | Rendering template #<Pathname:something.jpg> ("/something.jpg")

I think we can either:

1. Filter certain extensions here and not pass them to the template renderer
2. Filter certain extensions inside the resolver to match no files
3. Solve it in the libsass plugin by having it serve .scss files as well
4. Unregister Sass from Tilt