Allow a string to act as context

Fixes #321

mustache.js

@@ -251,7 +251,7 @@
       case '#':
         value = context.lookup(tokenValue);
 
-        if (typeof value === 'object') {
+        if (typeof value === 'object' || typeof value === 'string') {
           if (isArray(value)) {
             for (var j = 0, jlen = value.length; j < jlen; ++j) {
               buffer += renderTokens(token[4], writer, context.push(value[j]), template);

test/_files/nested_dot.js

@@ -0,0 +1 @@
+({ name: 'Bruno' })

test/_files/nested_dot.mustache

@@ -0,0 +1 @@
+{{#name}}Hello {{.}}{{/name}}

test/_files/nested_dot.txt

@@ -0,0 +1 @@
+Hello Bruno