support classes prop

[jedwards1211]

This allows passing a classes object, and extracts the current classes into the defaultProp value for it. This makes it easy to use with react-jss instead of importing the given styles.css:

import CircularProgress from 'react-circular-progressbar'
import injectSheet from 'react-jss'

const styles = {
  root: {
    width: '100%',
  },
  path: {
    stroke: '#3e98c7',
    strokeLinecap: 'round',
    transition: 'stroke-dashoffset 0.5s ease 0s',
  },
  trail: {
    stroke: '#d6d6d6',
  },
  {
    fill: '#3e98c7';
    fontSize: 20;
    dominantBaseline: 'middle',
    textAnchor: 'middle',
  },
}

export default injectSheet(styles)(CircularProgress)

[kevinsqi]

cool, thanks for submitting and for the clear explanation.

The classes prop is quite coupled with react-jss. Specifically, it has to be named "classes", or else injectSheet doesn't work (if I'm understanding correctly). I'm not a huge fan of the name "classes", because it seems very easy to confuse with className for users that don't use JSS.

More generally, the usefulness of this prop seems specific to react-jss as well. This prop doesn't help for radium/aphrodite/css-modules, which each require their own inline styling patterns, and it'd be nice for the component to be agnostic about styling libraries.

That being said, there's probably some general utility in adjusting the default classNames even if you don't use react-jss, and my concerns aren't too significant. So this looks good to go - thanks for another PR @jedwards1211!