chore(shell): add github shortcut so it shows shorter in the prompt

In the shell prompt when you were in a directory it would commonly be

`~s/github.com/AdeAttwood/Dotfiles|branch ⇒`

This mapping will shorten this down to try and make the prompt a bit
less intrusive when in a subdirectory of a project.

`~gh/AdeAttwood/Dotfiles|branch ⇒`

site-modules/core/files/oh-my-zsh/custom/shortcuts.zsh

@@ -22,3 +22,4 @@ mutt_scripts=~/.mutt/scripts
 #
 hash -d code=~/Code
 hash -d s=~/Code/src
+hash -d gh=~/Code/src/github.com