Add option to use Rhumb lines

[dankarran]

This pull request adds an option to use straight rhumb lines (and their equivalent bearing) instead of great circle lines. It's still useful in maritime navigation for example, even if it's not the most direct route. It's similar to the way the original leaflet.measure plugin worked.

The calculations are based on the handy website used for the other calculations.

As part of the update I've moved the arcpoints variable into the options array to allow it to be modified. It needs overriding when using rhumb lines, but this PR also then fixes the bearings and midpoint arrow placement compared to just setting the original _arcpoints variable to 2.

[dankarran]

I've just realised the distances are still using great circles (with the built-in Leaflet .distanceTo() method), so this needs updating still for consistency.