Feature request: Append to existing Git hooka

[philipjohn]

Instead of entirely overriding existing Git hooks, Husky should append itself as suggested by @sergiubologa. This would fix #171 which is experienced my many people and hampering workflows.

@typicode did suggest "It may be possible to do something with ~/.huskyrc though." but there is no more information on that and the solutions proposed in the rest of that issue don't seem to be feasible for adding to .huskyrc

@jimmynotjim offered to open a PR, so maybe that's a first step?