Add Template Helpers

Adds helper methods to a template via the following syntax:

```
template "name" do
  # singular method definition:
  helper(:method_name) { method_body }

  # inline module definition
  helpers do
    def method_name
      method_body
    end
  end

  # external module inclusion
  helpers(MyHelperModule)
end
```

Includes explicit tests for using helpers with partials, input validation in the Template resource and a little code cleanup.