add dotenv plugin (#4373)

plugins/dotenv/README.md

+# dotenv
+
+Automatically load your project ENV variables from `.env` file when you `cd` into project root directory.
+
+Storing configuration in the environment is one of the tenets of a [twelve-factor app](http://www.12factor.net). Anything that is likely to change between deployment environments–such as resource handles for databases or credentials for external services–should be extracted from the code into environment variables.
+
+## Installation
+
+Just add the plugin to your `.zshrc`:
+
+```sh
+plugins=(git man dotenv)
+```
+
+## Usage
+
+Create `.env` file inside your project directory and put your local ENV variables there.
+
+For example:
+```sh
+export AWS_S3_TOKEN=d84a83539134f28f412c652b09f9f98eff96c9a
+export SECRET_KEY=7c6c72d959416d5aa368a409362ec6e2ac90d7f
+export MONGO_URI=mongodb://127.0.0.1:27017
+export PORT=3001
+```
+`export` is optional. This format works as well:
+```sh
+AWS_S3_TOKEN=d84a83539134f28f412c652b09f9f98eff96c9a
+SECRET_KEY=7c6c72d959416d5aa368a409362ec6e2ac90d7f
+MONGO_URI=mongodb://127.0.0.1:27017
+PORT=3001
+```
+
+**It's strongly recommended to add `.env` file to `.gitignore`**, because usually it contains sensitive information such as your credentials, secret keys, passwords etc. You don't want to commit this file, it supposed to be local only.

plugins/dotenv/dotenv.plugin.zsh

+#!/bin/zsh
+
+source_env() {
+  if [[ -f .env ]]; then
+    source .env
+  fi
+}
+
+autoload -U add-zsh-hook
+add-zsh-hook chpwd source_env