Adding 'auto' option to 'outputDir'

Hi,

I've added an `auto` option for the `outputDir` setting, I'm sending a pull request in case you find it helpful.

Basically there are two typical folder structures that people use with less files:

 1. Less in `/projects/assets/less` and CSS in `/projects/assets/css`
 2. Less in `/projects/assets/css/less` and CSS in `/projects/assets/css`

My changes essentially do the following: check whether the parent folder is `css`. If yes, compile it there. If not, check whether there is a `css` folder in the parent folder, if yes, save it in that folder.

(Apologies for having two unnecessary commits in the history, I added some changes to the README file and then reverted them)

Let me know if this is useful (it is for me and my dev team).