add support for Template functions

[ccamel]

As far as I can see, it is not possible to define additional Template functions. For example, the following code defines some functions:

		if renderer.tmpl == nil {
			tmpl := template.New(name)
			tmpl.Funcs(map[string]interface{}{
				"add": func(n1, n2 int) int {
					return n1 + n2
				},
 ...

but it would be great to be able to add others (such as the functions provided by Sprig for instance), when creating a new renderer (NewRenderer).

[ccamel]

I can submit a PR if it can help. 😌
Just let me know.

[princjef]

I've put together a small change that adds a WithTemplateFunc option to the renderer. Let me know if that solves your issue

[ccamel]

@princjef Great! That's perfect. Thanks. 👍