Display an error message if run as root

*Not ready to be merged*

Fixes #35

I need some help with the error message. How can we briefly explain what is wrong with running `yo` as root without putting too much blame on the user? We should also point the user to a way to fix their installation, because chances are really good that they also run the rest of the toolchain with sudo.

/cc @stephenplusplus @sindresorhus