Add option to phaseThreshold to make it have the same value as animateCanvasDuration

[IanLunn]

In the Basic theme for example, the canvas is animated to reveal each step. To make a step only begin animating one the canvas has been animated, the phaseThreshold option muar be given the same value as animateCanvasDuration.

To save the developer having to change the phaseThreshold option manually, this can be inbuilt to Sequence.js. Currently phaseThreshold can be true (start after the current phase has animated-out), false (start immediately), or a custom value representing milliseconds. A new value such as auto can be used to make phaseThreshold the same as animateCanvasDuration. As this is a more intuitive approach, it'd probably work best to make auto the new default. When animateCanvas is disabled, the phaseThreshold value of auto can fallback to false to maintain backwards compatibility.