Add a render builtin

We use heavily `format` but we miss the ability to do named substitutions like we do in Python.  This is just a proof of concept (and it lacks documentation) of a `render` built-in. It uses [StringTemplate](http://www.stringtemplate.org/) to do substitutions like

```bash
"/foo" = render("hello <foo>", nlist("foo", "world"));
```

The new dependency is BSD-licensed. Before I continue polyshing it, I'd like to know if this approach is acceptable in the compiler.