Improve crosshair health display

[Calinou]

The new implementation has numerous upsides compared to the previous implementation:

• It indicates the player's health more accurately using a color gradient well-known among FPS players. Health above 100
  is indicated by tinting the crosshair green.
• It's always visible, even in screenshots. Due to lack of animation, it also avoids hiding the health status on every loop of the animation (since the crosshair became white).

The variable was renamed from crosshairflash to crosshairhealth to reflect the change.

[qreeves]

Changes requested, but I think a larger discussion needs to be done about whether the crosshair should be changing colours like this.

[q009]

I think this is fine, given it's gated behind a variable. So, it's up to each user's decision whether they wish to use this mechanism or not.

[qreeves]

I've looked at this more closely, and my issue with it is that it replaces the flash with the colour completely. I, personally, don't like the crosshair changing colour, which is why this was implemented as a flash instead that only alerts to low health.

Also the variable being used was only renamed in the usage.cfg and not in the source code. I suggest it be a bitwise variable: &1 = flash, &2 = colour. This way you can combine the two effects if you want as well.