Block difficulty configurable via queryString for blocks tab

[MrHmP]

I am providing difficulty for "blocks" tab from query string. This required minimum change in current code and it will help explain concept of difficulty while introducing nonce for a block.

If you want then I can take this concept forward for all tabs.

[anders94]

I'd rather not do this as passing those arguments on the URL seems bumpy. I'd rather it as a hidden "feature" and if so, on all pages that use difficulty.