This just returns the extra amount needed to add onto the string/array.
Also fixed up writeSLArray() not adding slack like it should have been. This wasn't an issue previously because the only place that currently uses this function is the password feature which is a fixed 16byte length, so it was already aligned. An upcoming commit will be using this, however, so I needed the alignment to be correct.
Finally, the read/write string functions were doing unnecessary conditional checks so I removed them. skipWrite() already does nothing if you tell it to skip 0, and adding 0 to readOffset won't do anything, so neither conditional is necessary at this point.
This password encoder was decompiled from the Android app then manually cleaned up and ported to Javascript. It ain't pretty, but it works.
I don't know that this is all necessarily the most correct or idiomatic way to implement these features, but I wanted to make sure the functionality got committed to help out people wanting to use this for smart home appliances (#1).
