Add plugin for pipenv (#6698)

- Add completion for pipenv
- Auto activate and deactivate for pipenv shell
- Add aliases for the most common pipenv commands

plugins/pipenv/README.md

+# Pipenv
+
+## Installation
+In your `.zshrc` file, add `pipenv` to the plugins section
+
+```
+plugins=(... pipenv ...)
+```
+
+## Features
+This plugin provides some features to simplify the use of Pipenv while working on ZSH. 
+- Adds completion for pipenv
+- Auto activates and deactivates pipenv shell
+- Adds short aliases for common pipenv commands
+  - `pch` is aliased to `pipenv check`
+  - `pcl` is aliased to `pipenv clean`
+  - `pgr` is aliased to `pipenv graph`
+  - `pi` is aliased to `pipenv install`
+  - `pidev` is aliased to `pipenv install --dev`
+  - `pl` is aliased to `pipenv lock`
+  - `po` is aliased to `pipenv open`
+  - `prun` is aliased to `pipenv run`
+  - `psh` is aliased to `pipenv shell`
+  - `psy` is aliased to `pipenv sync`
+  - `pu` is aliased to `pipenv uninstall`
+  - `pwh` is aliased to `pipenv --where`
+  - `pvenv` is aliased to `pipenv --venv`
+  - `ppy` is aliased to `pipenv --py`

plugins/pipenv/pipenv.plugin.zsh

+# Pipenv completion
+_pipenv() {
+  eval $(env COMMANDLINE="${words[1,$CURRENT]}" _PIPENV_COMPLETE=complete-zsh  pipenv)
+}
+compdef _pipenv pipenv
+
+# Automatic pipenv shell activation/deactivation
+_togglePipenvShell() {
+  # deactivate shell if Pipfile doesn't exist and not in a subdir
+  if [[ ! -a "$PWD/Pipfile" ]]; then
+    if [[ "$PIPENV_ACTIVE" == 1 ]]; then
+      if [[ "$PWD" != "$pipfile_dir"* ]]; then
+        exit
+      fi
+    fi
+  fi
+
+  # activate the shell if Pipfile exists
+  if [[ "$PIPENV_ACTIVE" != 1 ]]; then
+    if [[ -a "$PWD/Pipfile" ]]; then
+      export pipfile_dir="$PWD"
+      pipenv shell
+    fi
+  fi
+}
+chpwd_functions+=(_togglePipenvShell)
+
+# Aliases
+alias pch="pipenv check"
+alias pcl="pipenv clean"
+alias pgr="pipenv graph"
+alias pi="pipenv install"
+alias pidev="pipenv install --dev"
+alias pl="pipenv lock"
+alias po="pipenv open"
+alias prun="pipenv run"
+alias psh="pipenv shell"
+alias psy="pipenv sync"
+alias pu="pipenv uninstall"
+alias pwh="pipenv --where"
+alias pvenv="pipenv --venv"
+alias ppy="pipenv --py"