Add ability to pass content to tail

[mjoyce91]

This enables the ability to pass arbitrary content inside of the tail div. This is useful for situations where you want to display content relative to the tail, such as a tooltip. Below is an example.

[coveralls]

Coverage remained the same at 57.965% when pulling e093b47 on mjoyce91:tail-content into 24f60dc on react-component:master.

[mjoyce91]

Any chance we can get this merged?

[afc163]

3.1.0