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
```

ATM, this PR is incomplete, needs additional unit tests and input validation, but discussion of the feature/syntax is welcome.