commands ratelimit (energy system?)

[marado]

Implement a ratelimit system for the usage of certain commands (for instance, .cbuff).
This system can be implemented, for instance, using an energy pool (perhaps indexed to users' activity, like the one proposed to be measured in #63 ). Commands can have an "energy delta", associated, and while some commands can give you energy, others can take it away. If a user doesn't have the energy needed to execute a certain command, it won't be run (no one can have negative energy).