Feature Request: Add aliases for git diff commands

[hermitmaster]

Check list

• [X] I have read through the README
• [X] I have the latest version of forgit
• [X] I have searched through the existing issues

Environment info

• OS
  • [ ] Linux
  • [X] Mac OS X
  • [ ] Windows
  • [ ] Others:
• Shell
  • [ ] bash
  • [X] zsh
  • [ ] fish

Problem / Steps to reproduce

Add gds alias for git diff --staged. An alias for git diff --cached would be great also.

I'd happily open a PR for this.

Thanks for the great plugin!