Add WithNoEscape(bool) function to disable HTML escaping globally

[lambdalisue]

Handlebars support noEscape option and this PR add such feature with functional options pattern.

Now users can use WithNoEscape() like

source := `<div class="entry">
  <h1>{{title}}</h1>
  <div class="body">
    {{body}}
  </div>
</div>
`

ctx := map[string]string{
    "title": "All about <p> Tags",
    "body":  "<p>This is a post about &lt;p&gt; tags</p>",
}

tpl := handlebars.MustParse(source, handlebars.WithNoEscape(true))
result := tpl.MustExec(ctx)

fmt.Print(result)

I'm not sure if my changes are enough for noEscape option so let me know additional changes are required