func contentOfHelper(name string, help HelperContext) template.HTML {
	if s := help.Context.Get(name); s != nil {
		return s.(template.HTML)
	}
	return ""
}
ContentOf retrieves a stored block for templating and renders it.
/*
{{content_of "buttons"}}
package velvet
import (
	"html/template"
	"github.com/pkg/errors"
)
func contentForHelper(name string, help HelperContext) (string, error) {
	body, err := help.Block()
	if err != nil {
		return "", errors.WithStack(err)
	}
	help.Context.Set(name, template.HTML(body))
	return "", nil
}