1.8.0: Improve styling options

Added new abilities for styling handles/pips based on their indexes.

• a new [data-handle='n'] was added for each handle
• a new argument was added for formatter() and handleFormatter()
  • before: formatter = v => v, handleFormatter = v => v
  • now: formatter = (v,i) => v, handleFormatter = (v,i) => v
• bump versions of dependencies