homebrew: Load standard Homebrew environment variables into shell session.

However, guard for legacy Homebrew as `brew shellenv` is relatively new.

modules/homebrew/README.md

@@ -1,7 +1,16 @@
 Homebrew
 ========
 
-Defines Homebrew aliases.
+Defines Homebrew specific shell environment variables and aliases.
+
+Environment Variables
+---------------------
+
+Execute the following to list the environment variables loaded in the shell:
+
+```sh
+brew shellenv
+```
 
 Aliases
 -------

modules/homebrew/init.zsh

@@ -10,6 +10,16 @@ if [[ "$OSTYPE" != (darwin|linux)* ]]; then
   return 1
 fi
 
+#
+# Environmental Variables
+#
+
+# Load standard Homebrew shellenv into the shell session.
+# `brew shellenv` is relatively new, guard for legacy Homebrew.
+if (( $+commands[brew] )); then
+  eval "$(brew shellenv 2> /dev/null)"
+fi
+
 #
 # Aliases
 #