func NewTemplate(input string) (*Template, error) {
	hm, err := NewHelperMap()
	if err != nil {
		return nil, errors.WithStack(err)
	}
	t := &Template{
		Input:   input,
		Helpers: hm,
	}
	err = t.Parse()
	if err != nil {
		return t, errors.WithStack(err)
	}
	return t, nil
}
NewTemplate from the input string. Adds all of the
global helper functions from "velvet.Helpers".
package velvet
import (
	"github.com/aymerick/raymond/ast"
	"github.com/aymerick/raymond/parser"
	"github.com/pkg/errors"
)
type Template struct {
	Input   string
	Helpers HelperMap
	program *ast.Program
}