ENH,BUG: venv.py: merge settings when -e/--from-environ && --app,--venvstr,--venstrapp

[Wes Turner]

e.g. in ipython:

# this does not work (it's just --from-environ w/out the cmdline args)
!venv.py -e -xbash --venvstrapp=nbformat/nbformat

# workaround
!venv.py -xbash --venvstr=$VENVSTR --venvstrapp=nbformat

[Wes Turner]

| Src: https://github.com/westurner/dotfiles/blob/develop/scripts/venv_ipyconfig.py