Attempting to set Xdebug idekey from Vagrantfile

I don't think this PR is ready to pull in as is, because I can't get it to work - but because I don't know why it's not working, I was hoping if I sent it as a PR, you might be able to let me know what I've done wrong?

The general idea is that if someone sets an idekey in their `Vagrantfile`, it'll add the relevant line to the `xdebug.ini` file, else it won't do anything... right now, so far as I can tell, the not doing anything is winning.