Filter to compress HTML

Another attempt for #476, #1453. I still see issues related to whitespaces in HTML reported by Jekyll users. The document compression is a requested task, so why don’t we introduce a filter for that? Nokogiri is already on board, it can do many tricks.

The filter must not squeeze every byte, a solid reliable compression would cover most existing use cases. My implementation is based on http://stackoverflow.com/a/16427503

I would use the filter in the top-level layout:

```html
<!-- _layouts/compressed.html -->
{{ content | compress_html }}
```