Now capable of running a command from the GUI. #1

An arbitrary command can now be entered at the GUI and executed, but for some
reason only one command can be run; no further commands are executed after the
first command is run.
It works! You can run arbitrary commands from the GUI! #1

Note to self: use StyledDocument.getLength(), *not*
JTextPane.getText().length(). Investigate the different.

Issues that need to be addressed:
* The program hung when I ran "cmd" from within the GUI;
* "cd" does not work because a new "cmd" instance is created for each command
    that is run;
* Aliases fail to work for the same reason, so they have to be supported
    internally;
* Commands like "echo" and "dir" need to be implemented internally