Add clean option for Makefile

[nghi01]

Recently had a problem when forgotting to clean venv on my Windows machine leading to Permission Error. Looks like it's treating the venv created on my previous session as not belonging to me. Weird, but cleaning it and remaking the venv solves the issue, so I'm just creating an option for cleaning for convenience if anyone needs it.

[zimmermatt]

It would be better to not pull the supporting directory out from underneath an activated virtualenv.
Please add a guard to detect if the virtualenv is active and deactivate it if so.

Something like:

declare -F deactivate && deactivate
rm -rf venv

deactivate is a shell function that is inserted into the environment when source venv/bin/activate is run and removed when deactivate is called.

[nghi01]

Hmm, apparently Makefile cannot find the deactivate or the activate command to call. Based on this, it seems like every make command is executed in a new shell, which leads to our problem. One solution I found is we can use virtualenvwrapper as an extension, but we should decide together to use it or not.

[zimmermatt]

I've heard good things about virtualenvwrapper. Initially stuck with virtualenv since it's commonly used.

Other ones to look at before finalizing on that are the pyenv plugins for virtualenvs:
https://github.com/pyenv/pyenv-virtualenv
https://github.com/pyenv/pyenv-virtualenvwrapper

[nghi01]

Sounds good. I've cleared the clean-env and will work on the virtualenvwrapper on my own free time. Should be ready to get merged now so you can merge your coverage PR.