Added history of previously typed commands.
Commands are stored in the PHP Session as a simple list.  Javascript retrieves both the list, as well as converting the list into an easy to lookup hashmap that also records the frequency with which the commands (or subcommands) appear.
Added ability to arrow through history stack.
Pressing up or down in the input field will select the previous/next item from the history stack.  Position in the stack is remembered when submitting a command from the history, to make it easy to rerun a series of commands again.