Unliur chunk hooks

[dr-harper]

The unilur package seems to have some interesting chunk options for customising the appearance of custom blocks: http://koncina.github.io/unilur/articles/custom-boxes.html

Might be worth seeing whether we could use something like this in the knitrhooks package!

[nathaneastwood]

These are pretty neat. I probably won't get much time to work on the package between now and Christmas but hopefully I'll get some time over the holidays.