Adding optional support for standard naming of web root sub directories.

In our work flow, we have a git repository with a web folder in the root of the git repo. This web folder is what we want to specify as the web root. So when we create a folder under the parrot 'sites' folder such as 'newsite', we actually want the web root to point at 'newsite/web'.

Adding this optional configuration to the config.yml file in the same way as has been done for php version and box name seems like a sensible approach.

I've never sent a pull request before, so not sure if this is the right way to do so, let me know if not, it is about time I figured it out!

Hoping to start using the main parrot for all our local development environments and thus add to it's growth and improvement.

Many thanks.

Finn.