Add Pipfile support

[jayfk]

Safety already works with the currently active virtual environment, but it would be nice to to run:

safety check -r Pipfile

or

safety check -r Pipfile.lock

It's probably the best to wait for https://github.com/pypa/pipfile/pull/57 before working on this.

[katzdm]

Looks like pypa/pipfile#57 has been abandoned - Would it be worth revisiting this under that assumption, or was that PR an absolute blocker?

[gtsop]

What's the status of this?