Allow variable containing template

Clostache cannot render following template correctly.

mustache:
```ruby
puts Mustache.render("{{a}}-{{b}}", {:a => "{{b}}", :b => "c"})
# => "{{b}}-c"
```

clostache:
```clojure
(println (render "{{a}}-{{b}}" {:a "{{b}}" :b "c"}))
; => "c-"
```