Add image for the indeterminate state.

The image 'ic_indeterminate_check_box.png' is fetched from here:
https://github.com/google/material-design-icons/tree/master/toggle/ios/ic_indeterminate_check_box.imageset

Author: jav

img/ic_indeterminate_check_box.png

@@ -58,6 +58,9 @@ export default class CheckBox extends Component {
     }
 
     _renderImage() {
+        if (this.props.isIndeterminate){
+            return this.props.indeterminateImage ? this.props.indeterminateImage : this.genCheckedImage();
+        }
         if (this.props.isChecked) {
             return this.props.checkedImage ? this.props.checkedImage : this.genCheckedImage();
         } else {
@@ -66,7 +69,13 @@ export default class CheckBox extends Component {
     }
 
     genCheckedImage() {
-        var source = this.props.isChecked ? require('./img/ic_check_box.png') : require('./img/ic_check_box_outline_blank.png');
+        var source;
+        if (this.props.isIndeterminate) {
+          source = require('./img/ic_indeterminate_check_box.png');
+        }
+        else {
+          source = this.props.isChecked ? require('./img/ic_check_box.png') : require('./img/ic_check_box_outline_blank.png');
+        }
 
         return (
             <Image source={source} style={{tintColor: this.props.checkBoxColor}} />