Add health display

[OhPointFive]

This adds an optional health display below players' names, with the following syntax:

<scoreboard below-name="health"/>

There are a few issues, currently:

• Firstly, observers can see the health of other observers. I'm not sure if this is fixable — as far as I can tell either someone sees everyone's health or no one's.
• Until a player takes damage, their health is incorrectly shown as 0, which is fairly clearly not their actual health. As far as I'm aware, this is an issue with vanilla minecraft. This can be overcome by displaying a dummy value in place of health and manually updating it whenever a player's health changes (or possibly every tick) but that's fairly complicated and possibly has other issues.