Adds Homebrew and completion support to asdf plugin

* Modifies the search logic for asdf to include Homebrew when it is
installed.  The implementation is adapted from the pyenv plugin.  Per
feedback from @mcornella, asdf is either going to be in the
Homebrew directory or $HOME/.asdf.  Since no distributions appear
to package asdf, the other locations are unncecessary.  This new
assumption allowed the search logic to be simplified as well.

* Includes command completions for both regular and Homebrew installs.
This enhancement was inspired by PR $6045.

plugins/asdf/asdf.plugin.zsh

@@ -1,12 +1,33 @@
-# Find where asdf should be installed.
-ASDF_DIR="${ASDF_DIR:-$HOME/.asdf}"
+_homebrew-installed() {
+    type brew &> /dev/null
+}
 
-# Load asdf, if found.
-if [ -f $ASDF_DIR/asdf.sh ]; then
+FOUND_ASDF=0
+DEFAULT_ASDF_DIR="$HOME/.asdf"
+
+if [ -d $DEFAULT_ASDF_DIR/bin ] ; then
+    FOUND_ASDF=1
+    ASDF_DIR="$DEFAULT_ASDF_DIR"
+    ASDF_COMPLETIONS_DIR="$DEFAULT_ASDF_DIR/completions"
+fi
+
+if [ $FOUND_ASDF -eq 0 ] && _homebrew-installed ; then
+    ASDF_DIR="$(brew --prefix)/opt/asdf"
+    if [ $? -eq 0 -a -d $ASDF_DIR/bin ] ; then
+        FOUND_ASDF=1
+        ASDF_COMPLETIONS_DIR="$ASDF_DIR/etc/bash_completion.d/asdf.bash"
+    fi
+fi
+
+if [ $FOUND_ASDF -gt 0 ] ; then
+    export PATH=${ASDF_DIR}/bin:$PATH
     . $ASDF_DIR/asdf.sh
+    if [ -f $ASDF_COMPLETIONS_DIR/completions/asdf.bash ]; then
+      . $ASDF_COMPLETIONS_DIR/completions/asdf.bash
+    fi
 fi
 
-# Load asdf completions, if found.
-if [ -f $ASDF_DIR/completions/asdf.bash ]; then
-    . $ASDF_DIR/completions/asdf.bash
-fi
+unset FOUND_ASDF
+unset ASDF_DIR
+unset ASDF_COMPLETIONS_DIR
+