Make labelPosition support segmentIndex callback function

[mledwards]

I'd like for one of my labels to be nearer the end of the segment, but it doesn't look like labelPosition allows a callback function like the labelStyle does. I could create a PR for this?

Does anyone know a workaround?

[toomuchdesign]

Hi @mledwards, thanks for writing! Yes, I assume we could give it a try.
Would you like to open a PR to make labelPosition accept both a value and a callback function? Callback functions should be implemented with existing the functionProp utility.

Last but not least, docs and tests should be updated, too ✌️