Add pipenv --support instead of python -m pipenv.help

[erinxocon]

When homebrew is used to install python, the debug help command doesn't work.
pipenv --support will now be the flag to invoke when getting diagnostic information.

[techalchemy]

i'm guessing we want to keep .help too and just leave it as a thin wrapper around the new command

[erinxocon]

pipenv --support and python -m pipenv.help are both supported now with pipenv --support being preferred.