Group commits per author

[oussamabadr]

resolve #63

I've just implemented a new flag -G, when used with -a all, to display commits per author, e.g:

➜  git-standup git:(master) ✗ git-standup  -a all -G -d 900 -b master
/projects/git-standup
Kamran Ahmed (5):
      1b3904b - Add linting and formatting (Fri Apr 3 13:07:06 2020 +0400)
      10c9c59 - Run formatter (Fri Apr 3 13:21:59 2020 +0400)
      21fc963 - Run formatter (Fri Apr 3 13:23:02 2020 +0400)
      1b0e73b - Formatting and linting (Fri Apr 3 13:47:28 2020 +0400)
      5a707b1 - Add funding info (Mon Nov 23 01:16:56 2020 +0400)

Luca Guzzon (1):
      0352988 - Formatting and linting (Fri Apr 3 13:09:54 2020 +0400)

Pieter-Jan Baert (1):
      ce40800 - Updated usage function to clarify the -a option. (Thu Sep 30 15:18:12 2021 +0200)   

I submitted draft PR, so we can discuss the implementation and the flag name before updating the documentation and other files (if any).

I may also look if we can preserve the color of the output.

[JaKXz]

Sorry to comment on an old PR, but this would be pretty cool to merge! :) What is left to do?