api/examples: Simplify optparse use

- The default 'default' is None
- The default 'dest' is the option name, with - replaced with _
- The default 'action' is 'store'

Not coincidentally, these defaults are correct for most of our existing code.

(imported from commit 9c6078bd778324e08e1ca214a97281a7bdce08c2)

examples/send-message

@@ -33,22 +33,10 @@ Example: send-message --type=stream commits
 Example: send-message --site=https://zephyr.humbughq.com iago@humbughq.com
 """
 parser = optparse.OptionParser(usage=usage)
-parser.add_option('--site',
-                  dest='site',
-                  default="https://humbughq.com",
-                  action='store')
-parser.add_option('--api-key',
-                  dest='api_key',
-                  default=None,
-                  action='store')
-parser.add_option('--sender',
-                  dest='sender',
-                  default='othello@humbughq.com',
-                  action='store')
-parser.add_option('--type',
-                  dest='type',
-                  default='private',
-                  action='store')
+parser.add_option('--api-key')
+parser.add_option('--site',   default='https://humbughq.com')
+parser.add_option('--sender', default='othello@humbughq.com')
+parser.add_option('--type',   default='private')
 (options, args) = parser.parse_args()
 
 if len(args) == 0: