Add custom prefix config key

That way users can set custom command prefixes.

**Example** (from Alkali):
```
[Alkali] !config prefix ~
[CactusBot] Prefix updated to "~"
[Alkali] !cactus
<No response>
[Alkali] ~cactus
[CactusBot] Ohai, I'm CactusBot! :cactus
```