Dynamic legend content size

**Describe the bug**
When using elastic charts the legend should adjust based on content with a `max-width`/`max-height` for side/bottom legends.

**Screenshots**
![image](https://user-images.githubusercontent.com/19007109/63042579-f0bc8c00-be8f-11e9-9a85-86fa82cddb73.png)

See https://github.com/elastic/kibana/pull/33558#issuecomment-520847052

