Add first preliminary version of GIS

The first preliminary development version of GIS (git-identitiy-switcher).
Commandline argument parsing must be done manually because we want certain help
output formatting and argument combinations that are not or not easily possible
with python's argparse.
Add first half of 'show' functionality
Finish implementing 'show' subcommand
Implement list and add subcommands
Implement set subcommand
Implement the update subcommand
Implement rm (remove) subcommand
Improve output format for 'show'
Add check-git-user.sh