Add document class option "orange"

Add document class option that changes the alert colors to the complementary color of mpibluefont.

% Let the complementary color of mpibluefont be the alert color:
\definecolor{mpibluefont-comp}{HTML}{E85E3E}
\setbeamercolor*{alerted text}{fg=mpibluefont-comp}
\setbeamercolor*{block title alerted}{fg=white,bg=mpibluefont-comp}
\setbeamercolor*{block body alerted}{bg=mpibluefont-comp!30}

Reasoning: I can't distinguish the default alert color mpired from black. There are likely more. Also, I can not distinguish mpigreen from mpiblue, but I don't have a solution for that. I simply avoid mpigreen (at all cost) in all of my own presentations.