Allow images formats

Necessary for #343.

* They are identified by `vcodec=acodec='none'`
* These formats show as the worst in `-F`
* Any postprocessor that expects audio/video will be skipped
* `b*` and all related selectors will skip such formats
* This commit also does not add any selector for downloading such formats. They have to be explicitly requested by the `format_id`. Implementation of a selector is left for when #389 is resolved

yt_dlp/postprocessor/common.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import functools
 import os
 
 from ..compat import compat_str
@@ -67,6 +68,25 @@ class PostProcessor(object):
         """Sets the downloader for this PP."""
         self._downloader = downloader
 
+    @staticmethod
+    def _restrict_to(*, video=True, audio=True, images=True):
+        allowed = {'video': video, 'audio': audio, 'images': images}
+
+        def decorator(func):
+            @functools.wraps(func)
+            def wrapper(self, info):
+                format_type = (
+                    'video' if info['vcodec'] != 'none'
+                    else 'audio' if info['acodec'] != 'none'
+                    else 'images')
+                if allowed[format_type]:
+                    func(self, info)
+                else:
+                    self.to_screen('Skipping %s' % format_type)
+                    return [], info
+            return wrapper
+        return decorator
+
     def run(self, information):
         """Run the PostProcessor.