Fix #156 : Use  relative to root paths for all options in CLI

This pull request allow the root directory path to be up in the tree:

```coffeescript
groc:
  options:
    root:  "../"
    out:  "docs/"
    index:  "README.md"
  all:
    options:
      glob: ["README.md","lib/*.coffee", "css/*.scss"]
```

I edited the files object building of [`cli.coffee`](https://github.com/Plou/groc/blob/ad11ff6d94312b8887cf51a8a2eea69564573259/lib/cli.coffee) to apply the root, so the paths are now consistent with `project.coffee`

I had to edit the out argument type to make it relative to the root directory.