(patch): Small refactor of the internal animate method
Updated README with info about issues and contribution
(minor) line: Add support for setting x or y of start/end

You can now set only one of x or y for the start and end
properties, like this:
line.start.x = 50;
(minor) animation: Add support for nested properties

Nested properties will now work, like this:
line.animate({
  start: { x: 50, y: 30 },
}, { duration: 500 });
(patch) line: Fix length value when set at object creation

When the length property is set in the options object passed
in when the object is created, the positions must be
calculated correctly.
Update version number to 2.4.0