Add `hangar` format to release note script.

Hangar uses Markdown, so this is a very easy release note "conversion"
just like GitHub Releases.

scripts/extract-release-notes

@@ -26,7 +26,7 @@ def parse_args():
     parser.add_argument(
         '--format',
         '-f',
-        choices=['github', 'spigot', 'curse'],
+        choices=['github', 'hangar', 'spigot', 'curse'],
         help='the format to output the release notes in',
     )
 
@@ -80,6 +80,8 @@ def extract(target):
 def output(lines, fmt):
     if fmt == 'github':
         output_as_github_markdown(lines)
+    elif fmt == 'hangar':
+        output_as_hangar_markdown(lines)
     elif fmt == 'spigot':
         output_as_spigot_bbcode(lines)
     elif fmt == 'curse':
@@ -97,6 +99,15 @@ def output_as_github_markdown(lines):
     output_raw(lines[1:])
 
 
+def output_as_hangar_markdown(lines):
+    """
+    Hangar Versions use Markdown in the same format as GitHub Releases, so we
+    don't actually need to do anything else here either. Just strip the first
+    line so we don't get a duplicate header.
+    """
+    output_raw(lines[1:])
+
+
 def output_as_spigot_bbcode(lines):
     """
     Spigot uses BBCode for resource update descriptions. It's very similar to