Support colons in the parser's format_usage() output.

The split around ":" only needs to separate the automatic "usage:" prefix, not
any colons (for example in "HOST:PORT" strings) that happen to be in the
output. Split just once to ensure this.

argparse_manpage/manpage.py

@@ -169,7 +169,7 @@ def __init__(self, parser, _data=None, format='pretty'):
 
         self.formatter = self.parser._get_formatter()
         self.mf = _ManpageFormatter(self.prog, self.formatter, format=self.format)
-        self.synopsis = self.parser.format_usage().split(':')[-1].split()
+        self.synopsis = self.parser.format_usage().split(':', 1)[-1].split()
 
         self.date = self._data.get("date")
         if not self.date: