Option to choose who plays white when setting up a challenge

**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](https://www.chris-saylor.com/chessbot/gameplay/challenging.html).

Keep up the good work!

Start with the challenge command entry point and the linked challenging documentation; trace how command arguments are parsed and how the current color-selection logic works. Done means a challenge can explicitly assign white using an agreed syntax, existing behavior remains defined, and the documentation matches the implementation.

Written by the indexing model from the issue text.