Add concentration information to initiative tracker

[Kruptein]

In my games, we often forget about concentration checks, so I would like to add an easy quick identifier to the initiative tracker to show if someone has concentration.

[dthv]

Maybe this can be done simply by fixing #389 ?
One could then add an effect with e.g. the spell's name and e.g. a 'C' to show that there's aconcentration check needed. :)