!quo?: Staged quo's

[LinuxMercedes]

There ought to be a command that allows you to 'stage' a quote and later commit or reject it. This would allow people to check that their command is correct without having to adjust indices if the channel conversation keeps moving.

This would require a more centralized architecture for handling quotes. This would make it easier to implement additional quote databases and centralize filtering of noquo'd stuff.

(See #108 for slightly more discussion.)