SVG printing issue

* **C3 version**:0.4.17
* **D3 version**:3.5.17
* **Browser**:IE 11, Chrome 62, Firefox 54
* **OS**:Windows 10

Charts with lines do not print lines properly. In all browsers tested, in print preview the path is closed and filled with black.

This can be easily fixed using this css, per [this question](https://stackoverflow.com/questions/10213155/unclosed-svg-path-appears-to-be-closed):
```
path {
fill:none;
}
```

I'm not sure what knockon effects this will have for area charts, but presumably prefixing with .c3-line would solve the issue.