v0.4.0

• All tracks were migrated from D3/SVG→canvas. This results in a 4–5x speedup while panning.
• All tracks are now draggable.
• Alignments are displayed as pairs when applicable.
• Added a destroy method to safely tear down a pileup.
• Fixed a bug where VariantTrack didn't update after resize.
• Distributed bundles are built with NODE_ENV=production. This improves code size and ReactJS performance.
• ScaleTrack now includes arrow heads.
• Updated Flow from 0.12.0 → 0.16.0.