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

In the [Basic theme](http://demo.sequencejs.com/basic/) 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.