Configurable location for virtualenvs

[blarghmatey]

It would be great if there were a variable that can be overridden to either pass the complete path to a virtualenv directory, or a pattern used to discover virtualenvs. As it is right now there is a hardcoded set of options for locating the python executable for a given buffer, but it is rigid and doesn't match everyone's preferred means of organizing their Venvs.

It would also be great if it could automatically use the $WORKON_HOME value that gets set by tools such as virtualenvwrapper or pyvenv.

In conjunction with this, it would be helpful to add some documentation to specify how the python interpreter is selected and any ways that get added to configure that.

[seagle0128]

It's not clear to me. Which part is hardcoded? Do you mean lsp-python-ms-locate-python? I think the PR is better since I am not using virtualenvs locally.