Add support for near-zero rounding

[Spacarar]

When charts have values VERY close to 0 (or should be 0, but javascript is bad) it can sometimes show negative results when we may know the result is never negative.

This package could* provide support for near-zero rounding in the dcPlugin options that will wrap all the valueAccessor calls in a function that will round anything below a threshold value.

Perhaps** this could even provide other common valueAccessor wrapping functions.

• it definitely could, but I'm not sure it should or if that should be up to the end-user

** I'm really not sure what else would be all that useful, and see the first point.