terminal.py:  Added a new keyword argument to `Terminal.__init__()`:  `enabled_filetypes`.  If given (as an iterable of strings or `FileType` classes) these filetypes will be supported by the terminal instance.  If not given, 'all' file types will be enabled (e.g. PDFFile, PNGFile, JPEGFile, ...).  To completely disable support for special file types just pass an empty list, `None`, or `False`.