Rework [p]customcom edit for randomized responses

# Feature request

#### Select the type of feature you are requesting:

- [ ] Cog
- [X] Command
- [ ] API functionality

#### Describe your requested feature
Not sure if this is a bug or a feature request actually.

The customcommand edit command needs reworking. Currently it can only edit simple commands. But what if I want to add to a command that creates randomized responses? Perhaps a "[p]cc edit add" command? I have a randomized response command with over a dozen responses, and it's the type of command that grows overtime. I don't want to have to add every single response every time. Not sure what else there is to say about this. Let me know if this needs more explaining.