Cursed counting options

[ericthelemur]

!counting currently infers the step of the count (e.g. 3, 6, 9, 12, ... is valid as well as 1,2,3,4, ...). It would be quite fun to have some more options, maybe:

• infers start and step from the first two messages, so 5, 7 would be followed with 9, 11, 13, ... instead of limiting to multiples of the single number
• Allow counting some other sequences: primes, fibonacci, etc. - either needs to start with a specific command or infers from the first few messages