#39 Fix parsing of "cd" command

[Mark Saper]

Hi Warren,

On the Mac I'm using a convenient contextual menu
plugin called CopyPath. It copies a files path name
and then I can paste it into Pymol. It returns the
path surrounded by " (double quotes). This seems to
work in unix shells, but doesn't work in PyMOL. Would
handling double quotes be something that PyMOLS parser
could handle, or would that screw up the grammer for
the other commands?

Mark