Usage:
  GET /20c   => Celsius to Fahrenheit
  GET /90f   => Fahrenheit to Celsius
  GET /90    => Fahrenheit to Celsius (default)

Examples:
  /20c  -> 68
  /90f  -> 32.222222
  /90   -> 32.222222
  /-40  -> -40
  /36.6c -> 97.88