Allow mono8 recording and fix bug in local path

[Affonso-Gui]

1. Enable to save grayscale images with mono8 encoding (useful for mask images)
2. Fix bug when trying to save to local path with --image-topic:

Traceback (most recent call last):                                         
  File "/home/affonso/jsk_common_ws/src/jsk_common/jsk_rosbag_tools/scripts/bag_to_vid
eo.py", line 57, in <module>                
    main()                                      
  File "/home/affonso/jsk_common_ws/src/jsk_common/jsk_rosbag_tools/scripts/bag_to_vid
eo.py", line 53, in main                         
    audio_topic=args.audio_topic)                                                     
  File "/home/affonso/jsk_common_ws/src/jsk_common/jsk_rosbag_tools/python/jsk_rosbag_
tools/bag_to_video.py", line 88, in bag_to_video
    makedirs(osp.dirname(output_filepath))
  File "/home/affonso/jsk_common_ws/src/jsk_common/jsk_rosbag_tools/python/jsk_rosbag_
tools/makedirs.py", line 30, in makedirs
    raise OSError                                                          
OSError  
(OSError: [Errno 2] No such file or directory: '')  

[iory]

Thank you!