v5.0.0

• [BREAKING] #14 Removed the polyfill from the default builds, and shipping
  it instead as as separate module.
• [BREAKING] #21 Returning an object instead of an array, so that values not
  needed could be omitted.
• Improved ref handling:
  • #16 You can now pass in your own ref
  • The same hook instance can now be reused with different refs
  • The hook will no longer break if the ref is not immediately filled.
    (Anything other than an object with a .current value of an Element will
    be ignored.)
• Made defaults optional with the useDefaults option.
• #18 Added missing copyright notice in the MIT license.
• New package.json scripts to ease development
• Added throttle and debounce guides to the readme
• More tests