This commit makes a bunch of improvements:
1. Use format() for string formatting instead of the old style.
2. Stop returning stuff where we don't have to.
3. URL generation should not be done by the script, especially
since our eventual goal is for the Zulip server to automatically
generate a webhook URL. So the user should be able to supply
the URL in the config file. Plus, this makes it easier to test
the script with non-Zulip URLs (such as RequestBin).
4. Removed unnecessary arguments.
fa759fc97c