add support for `git unstage --all`

add the ability to unstage all ~~changed~~ staged files in a Git repository; the current implementation unstages files only the current working directory and below

gunstage/bin/git-unstage

Lines 26 to 28 in 4d6e288

    
           # run the same command against each argument, if any 
        
           # otherwise use `.` for everything in the current directory and below 
        
           for file in "${@:-.}"; do

	# run the same command against each argument, if any
	# otherwise use `.` for everything in the current directory and below
	for file in "${@:-.}"; do