Delete LESS files after conversion and fix mangling paths with "less"

I noticed that you closed #1 and #2 about consuming LESS files, but it looks like all of the other file conversion plugins are following the example set forth by metalsmith-markdown (metalsmith-sass, metalsmith-stylus, metalsmith-coffee, etc.).  I can understand not wanting to do it -- it definitely puts more power in the hands of the user of the plugin to delete files with metalsmith-ignore at their own discretion.

I also fixed replacing non-extension forms of `less` in paths (see the new test) and added in an `includePaths` option that acts as a convenience method for specifying paths relative to the source directory.  This is how the metalsmith-sass plugin does it, and it helps to expose a simpler option than sending in `paths` through the `parse` object.