add failing test for paths including `less`
delete LESS files and add includePaths option

instead of leaving the source files, metalsmith-less now removes them
- this matches the behavior of metalsmith-sass, metalsmith-stylus, and
  metalsmith-markdown

includePaths is a convenience option for specifying include paths
  relative to the Metalsmith source directory

fix mangling paths that include `less` outside of an extension
- this is intuitively the correct behavior