Allow user to set python virtual environment path

[chendaniely]

#39 mentioned changing a default reticulate::use_virtualenv("~/.venvs/pygradethis") we should make sure this gets incorporated into the python setup process so it can be changed as needed.

I think we're hard coding it right now, which is fine as we're developing but will need to abstract out this feature in the future for general use.

Maybe this is something we connect to #38