Add a better way to run standalone bots

[noemartinez]

zulip-run-bot currently supports running a bot in two ways:

1. By name, assuming it exists in the zulip_bots directory in a local clone of python-zulip-api
2. By an explicit path to the bot's handler .py file, regardless of where it lives

Neither of those two ways feels 100% well suited for running a standalone bot (a bot not meant to be contributed back or deployed as part of python-zulip-api) whose code and handler class is already deployed in the current python environment, for example as a standalone pip-installed package.

Can we add a better way to run this kind of bot? Perhaps by passing to zulip-run-bot an explicit module path to the python module that holds the bot's handler class, or something similar. Or perhaps by providing and advertising an easy to customize bit of code similar to what powers zulip-run-bot that can be pointed to the bot handler class, which I hear might already be in the works for docker-deployed bots, etc.

See this conversation for more context on this issue.