Uuhhhmm, add an in-place update flag

Like Ruby's `-i` flag, except without the confusing backup option. When I call it from vim, the vim integration code isn't smart enough to run it with the right filename, so then it runs in some random dir, and I can't use relative load paths. This is annoying b/c I'm making an example of how to use this lib, but I don't want a hard-coded fullpath of my local machine in my example.