Add support for relative numbering on numbered blocks (figure, example, etc)

[mojavelinux]

Add support for numbering blocks relative to the containing chapter (and perhaps section too). For example, the first figure in the first chapter should be 1.1 instead of 1. The second number should be reset when moving to the next chapter, making the first image in the second chapter 2.1.

More detail can be found in the following comment: #979 (comment)

[diguage]

This is a great idea.