Add chaining methods for numeric values

Currently, it is only possible with a regex to enable minimal and maximal values for numeric values. It would be great to have a syntax like that, as it is easier to read:

```elm
PropTypes.integer.min(1).max(10)
PropTypes.float.min(0).max(1)
```

 Or just one method
```elm
PropTypes.float.min(1)
PropTypes.integer.max(10)
```
