Allow summary/drill down of diff in drush

Fantastically helpful module, but would be a really handy feature to be able to view a summary and drilldown within drush feature-diff... I would propose the following changes:

1) Add an 'short' option that instead of giving you everything line by line, gives you something like git's diff --stat:

component: 4 insertions, 2 deletions

2) Add an optional argument that filter's the list of overrides by a particular component.

I've attached a possible patch that does both of these... pretty rudimentary and I'm sure can be vastly improved, but it served my purpose quickly and hopefully will help others...