Implement `useblock` tag to expand blocks multiple times

[christoph-heiss]

Hi!

So after using Tera I got to the same problem as discussed in #265, which is to expand a block multiple times. Since a new useblock tag was the proposed solution I though I'd implement that. The acutal core changes (136e1e1687fc665163c41cd299387190d2b714cd) aren't very invasive IMHO.

All test cases succeed, so existing users shouldn't be affected in any way due to being a completely new tag.

I added relevant test cases and updated the documentation where appriopriate. I hope that is ok as-is, but I'd be happy to revise it.

[jk-ozlabs]

@christoph-heiss are you still interested in this change? If not, I'm happy to continue your work here, if that's OK with you.