Enable piping html to browser without scripts.

Many browsers (including chromium) require that files end in the '.html'
extension in order to render them.
See <https://bugs.chromium.org/p/chromium/issues/detail?id=777737>.

When using `pipeto --format=mimepart --as_file`, on an html mime part, let's
automatically set the file extension.

This eliminates the need for additional user scripts referenced in
issues such as #789 and #1153.

alot/commands/thread.py

@@ -769,7 +769,9 @@ async def apply(self, ui):
 
             # Pass mail as temporary file rather than piping through stdin.
             if self.as_file:
-                with tempfile.NamedTemporaryFile(delete=False) as tmpfile:
+                suffix = {'html': '.html'}.get(mimepart.get_content_subtype())
+                with tempfile.NamedTemporaryFile(
+                        delete=False, suffix=suffix) as tmpfile:
                     tmpfile.write(mail.encode(urwid.util.detected_encoding))
                     tempfile_name = tmpfile.name
                 mail = None