Allow styling svg overflow with CSS.

When a chart's svg element is created, its overflow is set to `hidden` via an inline style [here](https://github.com/masayuki0812/c3/blob/4b34a1958edd686a934a193ad1d9f76fc14937bc/src/core.js#L242).

This makes customizing the svg's overflow behaviour harder than necessary; Essentially forcing us to either add `!important` to the style definition or override it with js after the element is created by c3.

Since the current behaviour is a reasonable default I propose making it optional via a config flag. This would allow disabling the inline overflow style like this:

``` js
{
overflow: false
}
```