Option to choose who plays white when setting up a challenge

[turboteddy]

Is your feature request related to a problem? Please describe.
I challenged player x, and I became white. After the game was ended, player x challenged me for a new game. I was playing white again.

Describe the solution you'd like
I'd like the ability to choose who should play white when a new challenge is set ut, e.g.:
@challenge @user white

Describe alternatives you've considered
Perhaps the syntax could be even a bit more intuitive
@challenge @user as white

Additional context
Whatever the current logic is for selection of color, it would be nice to have that logic described in the docs.

Keep up the good work!