Tooltip text color should be defined

The text color of tooltip should be defined. ie. When defining background color, define text color too. It's an ancient rule of CSS ;) But still true.

There is a white background rule, however text color is inherited from page CSS. When page has white text, this makes tooltip unreadable.

See attached image:


[JsFiddle](https://jsfiddle.net/rt8dLpny/1/)

I know that legend is not visible too, but c3 does not define text color at all for this - using browsers default - so no issue here.

* **C3 version**: 0.4.18
* **D3 version**: 3.5.17
* **Browser**: Chrome 62.0.3202.62
* **OS**: Linux Ubuntu 17.04