Support Named Package Categories

[matteius]

Thank you for contributing to Pipenv!

The issue

Fixes #4745

Description

Named package categories have many potential use cases -- including installing pre-requisite installation packages, system specific platform lock sections, using a single Pipfile for many sub-projects, and so much more!

This is a full feature PR to support --categories argument for most of the pipenv commands and code paths.

Example usages:

Single Category

pipenv install six --catetgories prereq

Multiple categories

pipenv install --categories="prereq packages"

Lock and Uninstall

pipenv lock --categories="prereq dev-packages" pipenv uninstall six --categories prereq