Version 0.10.1: morph_multi, new example and minor fixes/changes

- Added `morph_multi` to morph between two shape collections; refactored `_prepare_morph` and `morph` to support it; - `Line.leadin` and `Line.leadout` are now `int` (milliseconds); - New community example: **dangos** effect demonstrating `morph_multi` and animated shapes; - .gitignore updated to exclude generated `output.ass` files.