added "tweenState" module

[sc0ttj]

• added src/tweenState.js and related support files (src/raf.js, src/easings.js)
• new build process:
  • added rollup to bundle the source files into dist/ (see rollup.config.js)
  • added src/index.js to allow easier importing of multiple modules, if desired
• updated package.json: index.js is the file imported in Node by default
• now requires Node 10 or later (was Node 8)
• updated README
  • updated install instructions
  • added tweenState usage info