Add opt-in ping groups

[chatsonf]

Add the ability for the bot to manage ping groups so people can more easily organize things such as videogames.
For example:
@mommi addmeto Vidyagame would add me to a ping list named Vidyagame
@mommi removeme Vidyagame would remove me from the ping list
@mommi ping Vidyagame would ping either in chat or in PMs the people in Vidyagame

While not necessary it probably would be a good idea to have a command that lists the ping lists
@mommi pinglists would PM a list of currently existing lists (potentially with how many people are in them?)

[ghost]

working on it

[Exxion]

GitHub user MoMMI must be very confused about all these pings

[chatsonf]

it's been 19 days

[ghost]

I have no idea how to code this and my attempts were lacklustre at best.

[PJB3005]

Right, this is done now.