Added text option so you can have numbered/lettered markers

I wanted to create numbered and lettered markers rather than using fontawesome icons.  This fork adds support for a text option that you can use instead of icon when calling L.AwesomeMarkers.icon.  It also adds a textFormat option.  If textFormat = 'letter', a number is converted to a letter and displayed in the marker (0=A, 27=AA, etc).

Example usage:

num = 1;
var marker = L.marker([item.Latitude, item.Longitude], {
  icon: L.AwesomeMarkers.icon({
    text: num,
    textFormat: 'letter',
    color: 'blue'
  }),
});

// The letter used in the marker (B)
console.log(marker.options.icon.num2letter(num));