Fix resuming previous rips

Thanks to Freso for testing.

command/cd: Only call makedirs when dir does not exist
common/checksum: Support flac input files

Fixes #136

whipper/command/cd.py

@@ -338,8 +338,9 @@ Log files will log the path to tracks relative to this directory.
             else:
                 sys.stdout.write("output directory %s already exists\n" %
                                  dirname.encode('utf-8'))
-        print("creating output directory %s" % dirname.encode('utf-8'))
-        os.makedirs(dirname)
+        else:
+            print("creating output directory %s" % dirname.encode('utf-8'))
+            os.makedirs(dirname)
 
         # FIXME: turn this into a method