Support argcomplete as an optional dependency

[singingwolfboy]

argcomplete adds tab completion for Python CLIs build with the argparse module -- and it works with configargparse as well. This pull request sets up the Locust CLI to use argcomplete if it's installed, but if it's not installed, users can still use the Locust CLI without tab completion. In addition, this PR adds an "extra" requirement named "completion", so if users run pip install locust[completion], then Pip will install Locust and argcomplete.

[cyberw]

Nice! Can you add a note about this in the documentation?

[singingwolfboy]

Nice! Can you add a note about this in the documentation?

Sure, I just pushed a commit that does so. Unfortunately, this involves modifying the user's shell, and not everyone knows how to do that correctly, so I did a "happy path" description, and linked to the argcomplete documentation in case things don't go well. Does that work for you?

[cyberw]

Perfect

[cyberw]

@singingwolfboy this doesn't really work when there is no locustfile.py in the current directory. Can it be fixed?

Here's the trace output (after running export _ARC_DEBUG=1) when trying to autocomplete locust --hel

~ locust --helCould not find 'locustfile.py'. See --help for available options.

It is also kind of slow...

[cyberw]

Hmm... it really slows down "regular" tab completion (for things like filenames).

[cyberw]

@singingwolfboy I'm going to have to revert this because of the above issues being so bad. A fix would be welcome though...