Add a "cause of death" field to the Death object

[EverOddish]

Add a new string field to the Death model that describes how the Pokemon died. To start, we can just allow it to be set through the web interface. Maybe there is a good format for specifying it in the !rip command, but it has to be intuitive and easy.

Update the !deaths command to describe how each Pokemon died. Maybe make a new !death command to request the description of a specific Pokemon that died.