Gray color highlighting on click in iOS devices

When the line chart is opened in iPad, clicking on a point momentarily shows a gray overlay on the graph. Which is different from the way it works on desktop.

After digging further, I found that this the [functionality of safari running in OS X.](https://developer.apple.com/library/mac/documentation/AppleApplications/Reference/SafariWebContent/AdjustingtheTextSize/AdjustingtheTextSize.html#//apple_ref/doc/uid/TP40006510-SW5)

I also found that this can be disabled with a css and [created a pull request for the same](https://github.com/masayuki0812/c3/pull/1138). Please use it if it sounds like a fair fix