Add emoji-clock plugin

which displays the current time with half hour accuracy as an emoji
symbol, for example in the shell prompt.

plugins/emoji-clock/emoji-clock.plugin.zsh

+# ------------------------------------------------------------------------------
+#          FILE: emoji-clock.plugin.zsh
+#   DESCRIPTION: The current time with half hour accuracy as an emoji symbol.
+#                Inspired by Andre Torrez' "Put A Burger In Your Shell"
+#                http://notes.torrez.org/2013/04/put-a-burger-in-your-shell.html
+#        AUTHOR: Alexis Hildebrandt (afh[at]surryhill.net)
+#       VERSION: 1.0.0
+# -----------------------------------------------------------------------------
+
+function emoji-clock() {
+  hour=$(date '+%I')
+  minutes=$(date '+%M')
+  case $hour in
+    01) clock="🕐"; [ $minutes -ge 30 ] && clock="🕜";;
+    02) clock="🕑"; [ $minutes -ge 30 ] && clock="🕝";;
+    03) clock="🕒"; [ $minutes -ge 30 ] && clock="🕞";;
+    04) clock="🕓"; [ $minutes -ge 30 ] && clock="🕟";;
+    05) clock="🕔"; [ $minutes -ge 30 ] && clock="🕠";;
+    06) clock="🕕"; [ $minutes -ge 30 ] && clock="🕡";;
+    07) clock="🕖"; [ $minutes -ge 30 ] && clock="🕢";;
+    08) clock="🕗"; [ $minutes -ge 30 ] && clock="🕣";;
+    09) clock="🕘"; [ $minutes -ge 30 ] && clock="🕤";;
+    10) clock="🕙"; [ $minutes -ge 30 ] && clock="🕥";;
+    11) clock="🕚"; [ $minutes -ge 30 ] && clock="🕦";;
+    12) clock="🕛"; [ $minutes -ge 30 ] && clock="🕧";;
+     *) clock="⌛";;
+  esac
+  echo $clock
+}