Make Vertical Speed of Speedboat Configurable, default 2x

This PR pretty much came about because I built a boat elevator in my world but the speedboats go just as slow up it as the Vanilla boats do.

This PR makes the upward speed of the speedboat configurable. The modifier only takes effect if the speedboat is at least 90% covered/touching water (because otherwise I'd assume you'd get bouncy boats, which would be fun but probably not a proper feature).

There is a new entry in the configuration GeneralStuff (general-stuff.cfg) to allow the modification of this value.

By default, I have set it to 2.0x. You may wish to change this default, but I'll leave that to you.

Cheers.