Bmson 1.0 support plan

[dtinth]

• Implement basic bmson 1.0 support Basic support for bmson 1.0 bmson#1
  
  Basic support means no new features that require big changes to game logic (e.g. mix notes, new slicing algorithm (current one is almost good enough))
• Update Bemuse to use new version of bmson
• Update bemuse-tools to use new version of bmson
• Refactor bms-js to allow metadata on each keysound file (breaking change, so so semver-major)
• Make bemuse work with new bms-js
• Make bemuse work with multiple keysounds per note
• Update bmson to create one keysound per slice
• Update bmson to support bmson 1.0 format