Merge: Popcorn templates

Merge: Popcorn templates

# Template rendering for popcorn

## Basic templates

Use TemplateString to render really basic templates that just need macro
replacements.

Example:
~~~nit
class TemplateStringHandler
        super Handler
	redef fun get(req, res) do
		# Values to add in the template
		var values = new HashMap[String, String]
		values["USER"] = "Morriar"
		values["MYSITE"] = "My super website"

		# Render it with a shortcut
		res.template_string("""
			<h1>Hello %USER%!</h1>
			<p>Welcome to %MYSITE%.</p>
		""", values)
	end
 end
 ~~~

 For larger templates, you can also use external files (makes your Nit code cleaner):
 ~~~nit
 class TemplateFileHandler
	super Handler

	redef fun get(req, res) do
		# Values to add in the template
		var values = new HashMap[String, String]
		values["USER"] = "Morriar"
		values["MYSITE"] = "My super website"

		# Render it from an external file
		res.template_file("example_template.tpl", values)
	end
 end
 ~~~

 ## Using pug templates

 Pug is a templating format provided by the external command `pug`.
 For complex templates that need conditional or loop statements, pug can be a solution.

 See the pug syntax here: https://pugjs.org/api/getting-started.html

 ~~~nit
 class PugFileHandler
	super Handler

	redef fun get(req, res) do
		# Values to add in the template
		var json = new JsonObject
		json["user"] = "Morriar"
		json["mysite"] = "My super website"

		# Render it from an external file
		res.pug_file("example_template.pug", json)
	end
 end
 ~~~

Pull-Request: #2449
Reviewed-by: Jean Privat <jean@pryen.org>