Allow more control over bossbar colors

[FireML]

Adds config options for the time colors:

bossbar:
  # The abbreviations used for different units of time
  hour-color: '&a'
  hour: h
  minute-color: '&b'
  minute: m
  second-color: '&c'
  second: s
  # For translating the "left" at the end e.g. "5s left" -> "5s kvar"
  remaining: left

The defaults are all &r so nothing is broken in older configs.