Allow a value to be specified for increment and decrement

[devdazed]

Currently increment and decrement assume that the only value one would want to increment by is 1. It would be nice to add an optional value and default it to 1.

[yannmh]

Thanks for your suggestion @devdazed,

That's something we are working on for the upcoming release 🚢, c.f. #27.

[degemer]

Implemented in #33, will be part of 2.1.0.