Add ol.coordinate.format function

Return a formatted coordinate. Example usage:
```javascript
ol.coordinate.format([0.01 ,12.14], "{x}, {y}", 2)
"0.01, 12.14"

ol.coordinate.format([0.01 ,12.14], "lat: {y} / lon: {x}", 1)
"lat: 12.1 / lon: 0.0"
```