Support for different character tags in the templates

Supports various characters tags in the templates according to TemplateLoader.

Example:

mail.html:
```html
[[define "show"]]
ok
[[end]]
```

invoice.html:
```html
<<define "invoice">>
ok
<<end>>
<<template "invoice">>
```

app.go:
```go
func (c App) Mail() r.Result {
	tl := rebel.NewTemplateLoader([]string{"/project/name/views/mail"}, "/project/name/views/mail", "[[ ]]")
	tl.Refresh()
	tpl, err := tl.Template("mail.html")
	if err != nil {
		return c.RenderError(err)
	}
	return &r.RenderTemplateResult{
		Template:   tpl,
		RenderArgs: c.RenderArgs,
	}
}

func (c App) Invoice() r.Result {
	tl := rebel.NewTemplateLoader([]string{"/project/name/views/invoice"}, "/project/name/views/invoice", "[[ ]]")
	tl.Refresh()
	tpl, err := tl.Template("invoice.html")
	if err != nil {
		return c.RenderError(err)
	}
	return &r.RenderTemplateResult{
		Template:   tpl,
		RenderArgs: c.RenderArgs,
	}
}
```