Better associative array handling

Bug:  Did not properly detect the length of an array in mustache-test
because I neglected to use `[@]` after the array name.  This closes
issue #6.

Feature:  Made {{ARRAY_NAME}} implode array values with commas.

Feature:  Added an associative array demo script.

demo/associative-arrays

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+declare -A DATA
+DATA=([one]=111 [two]=222)
+
+cat <<EOF | . ~/bin/mo
+Accessing data directly:
+    DATA: {{DATA}}
+    One: {{DATA.one}}
+    Two: {{DATA.two}}
+
+Things in DATA:
+{{#DATA}}
+    Item: {{.}}
+{{/DATA}}
+EOF