[commands] Add more i18n properties for HelpFormatter

removed fixed strings "Commands:" and help page ending note.
and added properties modify these strings.
default behavior is not changed. fix #1886

discord/ext/commands/formatter.py

@@ -142,11 +142,20 @@ class HelpFormatter:
     width: :class:`int`
         The maximum number of characters that fit in a line.
         Defaults to 80.
+    commands_heading: :class:`str`
+        The command list's heading string used when the help command is invoked with a category name.
+        Useful for i18n. Defaults to ``"Commands:"``
+    no_category: :class:`str`
+        The string used when there is a command which does not belong to any category(cog).
+        Useful for i18n. Defaults to ``"No Category"``
     """
-    def __init__(self, show_hidden=False, show_check_failure=False, width=80):
+    def __init__(self, show_hidden=False, show_check_failure=False, width=80,
+                 commands_heading="Commands:", no_category="No Category"):
         self.width = width
         self.show_hidden = show_hidden
         self.show_check_failure = show_check_failure
+        self.commands_heading = commands_heading
+        self.no_category = no_category
 
     def has_subcommands(self):
         """:class:`bool`: Specifies if the command has subcommands."""
@@ -317,7 +326,7 @@ class HelpFormatter:
             cog = tup[1].cog_name
             # we insert the zero width space there to give it approximate
             # last place sorting position.
-            return cog + ':' if cog is not None else '\u200bNo Category:'
+            return cog + ':' if cog is not None else '\u200b' + self.no_category + ':'
 
         filtered = await self.filter_command_list()
         if self.is_bot():
@@ -332,7 +341,7 @@ class HelpFormatter:
         else:
             filtered = sorted(filtered)
             if filtered:
-                self._paginator.add_line('Commands:')
+                self._paginator.add_line(self.commands_heading)
                 self._add_subcommands_to_page(max_width, filtered)
 
         # add the ending note