Virtual environment considerations for loadable DSI plugins

[qwofford]

We should consider whether it makes sense to associated virtual environments with plugins. This is a low priority consideration. It should be driven by a need which we do not see yet.

[jpulidojr]

With solving Issue #148 , merging PR #149 , solving #150 , and merging PR #151 , we should soon be able to associate virtual environments with specific plugins and containerize subsets of plugins into specific venv's depending on the installation of pip modules in that environment.