Detect and handle CD-R discs (#154)

* Use cdrdao to detect CD-Rs and continue ripping only if the argument --cdr is passed.

* Get and display disc format from MusicBrainz.

* Add info about CD-R detection to logger

* Remove Musicbrainz medium format info

* add cdrdao command to logger

* Fix for PEP8

* Fix line break

whipper/command/cd.py

@@ -143,6 +143,13 @@ class _CD(BaseCommand):
                                 "--unknown not passed")
                 return -1
 
+        self.program.result.isCdr = cdrdao.DetectCdr(self.device)
+        if (self.program.result.isCdr and
+                not getattr(self.options, 'cdr', False)):
+            logger.critical("inserted disc seems to be a CD-R, "
+                            "--cdr not passed")
+            return -1
+
         # FIXME ?????
         # Hackish fix for broken commit
         offset = 0
@@ -300,6 +307,11 @@ Log files will log the path to tracks relative to this directory.
                                  action="store_true", dest="unknown",
                                  help="whether to continue ripping if "
                                  "the CD is unknown", default=False)
+        self.parser.add_argument('--cdr',
+                                 action="store_true", dest="cdr",
+                                 help="whether to continue ripping if "
+                                 "the disc is a CD-R",
+                                 default=False)
 
     def handle_arguments(self):
         self.options.output_directory = os.path.expanduser(