Expand on from/to functionality

Currently you can include the following in your config to animate the progress bar's color between a starting and end point:
```
from: { color: 'red' },
to: { color: 'green' },
step: (state, bar) => {
bar.path.setAttribute('stroke', state.color);
}
```
CSS3 animations support this `from` and `to` style, but also allow for a percentage based way to define the various states, so you could have more than two, and they could be distributed arbitrarily.
```
0%: { color: 'red' },
50%: { color: 'yellow' },
100%: { color: 'green' },
step: (state, bar) => {
bar.path.setAttribute('stroke', state.color);
}
```
Is there an easy way to accomplish this before I get around to submitting a PR?

No source file or test is named. Start by tracing how the existing from, to, and step configuration is handled in progressbar.js, then compare it with the requested percentage-based states. Done means multiple arbitrarily placed percentage states work while the current from/to form remains supported.

Written by the indexing model from the issue text.