Add define to options

I've run into the problem of having to pass a variable to my stylus files via my gulp tasks every now and then and think it's probably something other people want to do. To be honest, I don't know why we can't get this plugin up to par with the grunt equivalent (https://github.com/gruntjs/grunt-contrib-stylus). Feature parity would be ideal. 

Unfortunately, I don't have time to add all the other options but it should be fairly simple.