Show the list of supported file types in the manual and --help output. We can...

Show the list of supported file types in the manual and --help output. We can copy-paste this into the website, at least. Thanks to Jonathan Dowland for the suggestion. (Closes: #893443)

diffoscope/comparators/__init__.py

@@ -126,3 +126,10 @@ class ComparatorManager(object):
                 ))
 
         logger.debug("Loaded %d comparator classes", len(self.classes))
+
+    def get_descriptions(self):
+        for x in self.classes:
+            try:
+                yield x.DESCRIPTION
+            except AttributeError:
+                pass

diffoscope/main.py

@@ -73,7 +73,7 @@ class BooleanAction(argparse.Action):
 def create_parser():
     parser = argparse.ArgumentParser(
         description='Calculate differences between two files or directories',
-        add_help=False)
+        add_help=False, formatter_class=HelpFormatter)
     parser.add_argument('path1', nargs='?', help='First file or directory to '
                         'compare. If omitted, tries to read a diffoscope diff from stdin.')
     parser.add_argument('path2', nargs='?', help='Second file or directory to '
@@ -265,6 +265,15 @@ def create_parser():
     return parser, post_parse
 
 
+class HelpFormatter(argparse.HelpFormatter):
+    def format_help(self, *args, **kwargs):
+        val = super().format_help(*args, **kwargs)
+        val = '{}\n\nfile formats supported:\n'.format(val)
+        for x in sorted(ComparatorManager().get_descriptions(), key=str.title):
+            val = '{}{}- {}{}.\n\n'.format(val, ' ' * 24, x[0].upper(), x[1:])
+        return val
+
+
 class RangeCompleter(object):
     def __init__(self, start, end=0, divisions=16):
         if end < start: