Enable showing segment labels only on hover

[jacktreg]

Thought of this feature when writing #193 . It seemed like it would improve the UX to allow users to optionally hide segment labels unless the speedometer SVG is hovered over, similar to how a tooltip would behave. The inspiration was that with both segment labels and stop labels the speedometer starts to become a bit noisy.

[palerdot]

Thanks for the PR! I will take a look at this when time permits and consider merging it to master.