Add plenv plugin

Based mostly from pyenv plugin.

plugins/plenv/plenv.plugin.zsh

@@ -0,0 +1,31 @@
+_homebrew-installed() {
+    type brew &> /dev/null
+}
+
+_plenv-from-homebrew-installed() {
+    brew --prefix plenv &> /dev/null
+}
+
+FOUND_PLENV=0
+plenvdirs=("$HOME/.plenv" "/usr/local/plenv" "/opt/plenv")
+if _homebrew-installed && _plenv-from-homebrew-installed ; then
+    plenvdirs=($(brew --prefix plenv) "${plenvdirs[@]}")
+fi
+
+for plenvdir in "${plenvdirs[@]}" ; do
+    if [ -d $plenvdir/bin -a $FOUND_PLENV -eq 0 ] ; then
+        FOUND_PLENV=1
+        export PLENV_ROOT=$plenvdir
+        export PATH=${plenvdir}/bin:$PATH
+        eval "$(plenv init - zsh)"
+
+        function plenv_prompt_info() {
+            echo "$(plenv version-name)"
+        }
+    fi
+done
+unset plenvdir
+
+if [ $FOUND_PLENV -eq 0 ] ; then
+    function plenv_prompt_info() { echo "system: $(perl -Mversion -e 'print version->parse($])->normal' 2>&1 )" }
+fi