add g:jedi#virtualenv_path to specify virtualenv

[theoremoon]

Hello.

I added g:jedi#vritualenv_path to specify the virtual environment like $VIRTUAL_ENV, but this will work even after Jedi is started.

Would you please merge if you think this feature is good.

The following script is my usage.

" set the pipenv's environment for completion when open .py file
function! s:setPipenvPath()
  let pipenv_dir = expand('%:p:h')
  if !filereadable(l:pipenv_dir . '/Pipfile')
    return
  endif

  let venv_path = trim(system(printf("sh -c 'cd %s; pipenv --venv'", pipenv_dir)))
  let g:jedi#virtualenv_path = venv_path
endfunction

autocmd FileType python :call s:addPipenvPath()

[blueyed]

I think we should finish #836 instead.

[theoremoon]

@blueyed I see. Thank you :)

[blueyed]

Thanks for your PR in the first place anyway, of course!

Please give #836 a try/review.