Added option to use the main scoreboard for the scoreboard healthbar

[tajobe]

Relates to discussion in #19

The main caveat with using the main scoreboard (and the reason I chose to not make it default) is that the main scoreboard is assigned to players by default so the health will always show, rather than for 5s after taking damage (assuming there isn't already an objective in that slot).

[Puremin0rez]

Scoreboard API in general is a massive hack, which is the same issue I ran into with my own code over at https://github.com/Puremin0rez/Dethemium/pull/22 and was discussed

"Main" scoreboard is good for compatibility because it interacts directly with the vanilla scoreboard, meaning you will always have your scoreboard active even if another plugin is creating their own scoreboard layered on top of it. Assuming they are not messing with your objectives or display slots, "main" is a good compatibility choice. "new" scoreboards are the correct thing to do in a vacuum as you don't want to be dealing with vanilla scoreboards directly really, but in reality when you have multiple scoreboard plugins you're going to hit a wall with that since there can only really be a single "new" scoreboard at a time.

Default off is the correct choice, since a new scoreboard is the correct behavior by default IMO. It should only be switched for compatibility reasons, which is exactly as it is in this PR.