A few users have complained about how hard it is to find the
Trello script on their systems after installing the `zulip`
package. One way to solve this issue is to make the Trello
script a part of the exported console scripts in the zulip
package, which would mean that the user would not have to
navigate to a particular directory in order to find the script,
but could run it from a terminal directly.
However, to make this happen, we need to minimize the script's
reliance on external configuration files, because we don't want
the user to have to figure out where the config file lives.
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.
