`pipenv run ...` searches for a Pipfile itself, but doesn't search far
enough, creating a Pipfile in the current directory. Instead, we run the
command from the pipenv project root so it doesn't have to search.
+ Rewritten +conda support
+ Adds +pyenv and +pyvenv flags with support.
+ New +ipython flag to enable ipython REPL support
+ Added pipenv support. This is the new default, instead of pyenv, and
isn't hidden behind a module flag because it is officially endorsed by
python.
Addresses #736
