Add a new {% render %} tag

Example:

```
// the_count.liquid
{{ number }}! Ah ah ah.

// my_template.liquid
{% for number in range (1..3) %}
  {% render "the_count", number: number %}
{% endfor %}

Output:
1! Ah ah ah.
2! Ah ah ah.
3! Ah ah ah.
```

The `render` tag is a more strict version of the `include` tag. It is
designed to isolate itself from the parent rendering context both by
creating a new scope (which does not inherit the parent scope) and by
only inheriting "static" registers.

Static registers are those that do not hold mutable state which could
affect rendering. This again helps `render`ed templates remain entirely
separate from their calling context.

Unlike `include`, `render` does not permit specifying the target
template using a variable, only a string literal. For example, this
means that `{% render my_dynamic_template %}` is invalid syntax. This
will make it possible to statically analyze the dependencies between
templates without making Turing angry.

Note that the `static_environment` of a rendered template is inherited, unlike
the scope and regular environment. This environment is immutable from within the
template.

An alternate syntax, which mimics the `{% include ... for %}` tag is
currently in design discussion.

test/test_helper.rb

@@ -121,3 +121,17 @@ class ErrorDrop < Liquid::Drop
     raise Exception, 'exception'
   end
 end
+
+class StubFileSystem
+  attr_reader :file_read_count
+
+  def initialize(values)
+    @file_read_count = 0
+    @values = values
+  end
+
+  def read_template_file(template_path)
+    @file_read_count += 1
+    @values.fetch(template_path)
+  end
+end