Add functionality to consume less files after compilation

Previously, compiling this directory:

```
./src/less/test.less
     /index.md
```

would result in this build:

```
./build/css/test.css
       /less/test.less
       /index.html
```

Note that the *.less files are passed along into the build as well as the compiled *.css files. However, with this change, metalsmith-less will consume the *.less files during compilation so they don't show up in the build.