Skip default xpi

This is an alternative to #229 - my proposal here building on the jpmignore work is to always add the generated xpi file name to the ignore list.

Some questions - is this better than creating a jpmignore file at init? I think so, because if the developer changes the id in package.json, the xpi file name created at build time will be different than what we put in .jpmignore at init. cc @erikvold @Gozala @jsantell