Synchronously updating the config could stop the spring

[appsforartists]

In #4, I added a call to this._step() inside updateConfig(). That's needed to ensure that the default values in updateConfig() are current before the spring is reset. However, it's also the only place that _step() gets called synchronously.

This introduces the possibility that changes to the spring config could stop the spring before it even starts. For instance

spring.updateConfig({ toValue: 0 });
spring.start();
spring.updateConfig({ fromValue: 1 });

Previously, the config wasn't evaluated until the next frame, so this would cause a transition from 1 to 0. However, now that it can be evaluated synchronously, when both fromValue and toValue are 0, which would stop the spring before fromValue: 1 is evaluated.

I'm not sure if this is a desirable behavior or not, but since it's a change, I thought I would at least document it. If it causes problems in the future, I'll address it.

One potential solution: add a flag to _evaluateSpring to suppress notifications. When called during updateConfig, notifications would be suppressed, which would prevent onUpdate, onStop, and stop() from being called.