Multiple cdn data

[solcre]

It would be awesome to be able to specify more than one cnd-data and try to replace components in order. For example if I add: google-cdn-data and cndjs-data it will try to replace references that are hosted in google first, and if they can't be found in google then try cdnjs.

[david-mohr]

I have submitted PR #37 to implement this feature.