Support less' rootpath and relativeUrls options

- The rootpath and relativeUrls options were added to less in version
1.3.2
- rootpath exposed as urlRootPath to make more sense along with the
other available recess options than just "rootpath" would. Also,
camelCase.

This allows properly compiling Less files that import from a subfolder with Less files referencing image or font files in that same subfolder, without having to flatten the file hierarchy, by updating urls to be relative to the main Less file's location. The same functionality was used to fix up browser-side loading when updating from Less 1.3.1 to 1.3.3 (The breaking change happened in 1.3.2).