[feature] pagination for posts summary

Currently there is a way to do `prev` / `next` but not `0`, `1`, `2`, `...`
```html

I would like to add numbers between `prev` and `next` like in the example below.

To make it possible we need to add data to Selmer with page index, URL, is page active.

Here is a Hugo example:
```html
{{ if gt $paginator.TotalPages 1 }}

• 
  
  «
  
  

{{ range .Paginator.Pagers }}

• {{ .PageNumber }}

{{ end }}

• 
  
  »
  
  

{{ end }}
```

This is the part which Cryogen doesn't have:
```html
{{ range .Paginator.Pagers }}