Slight climbing motion modifier

[eric-ycw]

Similar to #433, a climbing modifier would be nice. By climbing, I didn't mean you can climb a whole wall, just a small distance, because climbing up stairs, especially long ones, is rather annoying. You have to jump every single time you go up a step. With this, you can just hold W and walk up the stairs with ease without having to jump every time.

[LunaSolVT]

We covered this in 2D for my game design class. It shouldn't be that difficult. :p