Turn penalties

Hi there,

I'm using this library to draw a network graph on top of a grid, and I was wondering if you could suggest how to best implement a "[turn penalty](https://github.com/qiao/PathFinding.js/pull/150/files)", to avoid a stair-casing effect, using the `distance` and `heuristic` functions.

Thanks!

Start by reading the library's distance and heuristic functions, then compare the linked PathFinding.js turn-penalty implementation. Define how turn history would affect path costs and what behavior should eliminate stair-casing; the issue does not name a file or test that verifies completion.

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