Support negative activity levels

To support negative activity levels and counts the minLevel prop was added. See documentation and #122.

<ActivityCalendar
  data={data}
  minLevel={-6}
  maxLevel={3}
  labels={{
    legend: { less: '-6', more: '3' },
  }}
  theme={{
    light: ['red', 'hsl(0, 0%, 92%)', 'green'],
  }}
/>