Added force parameter to allow overwriting files

I think it would be a very useful addition for broccoli-cli to have a `--force` flag for `broccoli build dist`, so you don't always have to manually delete the folder. This addition keeps the old behaviour in place, and just adds the possibility of a third parameter `true/false`. 
If it is false or undefined, we have the original behaviour.
If it is true, then the potential `EEXIST` error from `mkdirSync` gets ignored and files get written with the `w` flag instead of `wx`.