feat: arbitrary post push hooks

[ghostsquad]

since git doesn't allow for arbitrary post-push hooks, it would be cool if gacp supported it.

would indirectly solve #45 and #44

[vivaxy]

Great idea.
We can have some gacp hooks.

[ghostsquad]

@vivaxy this still needs documentation

[vivaxy]

@ghostsquad See here: f0d5adf#diff-04c6e90faac2675aa89e2176d2eec7d8R49

If you'd like to config globally, you can create a .gacprc file in your home folder(~/.gacprc). If you'd like to use this hook in a certain project, you can add a .gacprc file in your project root.

The config can be like:

{
  "hooks": {
    "postpush": "${your command}"
  }
}