Latte: improved HTML comments escaping and tokenization

This solves problem described in https://github.com/nette/nette/commit/bb466dc37680172addc6419d0f76adf031def2ff#commitcomment-5235831 and aligns the HTML comments processing in general to the [HTML specification](http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html).

Note: `<!-->` is invalid comment and causes parse error but it [switches state back to data state](http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#comment-start-state).