[#84572026] added StringExtensions#letter

- looks cleaner
- used in lib/pgit/command.rb
[#84572026] added the PGit::Command#to_s method to print a command
[#84572026] added command list
[#84572026] clean up comments
[#84572026] added PGit::Command::Application
[#84572026] added .agignore to remove clutter from search results
[#84572026] remove comment that don't apply anymore
[#84572026] execute commands when commands exist