v0.2.0

• Changed force.initialize to take an array of nodes, not the simulation.
• You can now pass a radius to d3.forceCollide.
• The links argument to d3.forceLink is now optional and defaults to the empty array.
• The nodes argument to d3.forceSimulation is now optional and defaults to the empty array.
• Removed the start event from simulation.on.
• Fixed simulation.start, simulation.stop and simulation.tick; you can now tick manually.