[iOS] Support to use loading-indicator separately.

* feature: Support to use loading-indicator separately. Use animating property to control.
* doc pr: apache/incubator-weex-site#363
* demo: http://dotwe.org/vue/15b0ac96db6e1837ef5c27c0345b7fb6

Author: huanglei.hl

ios/sdk/WeexSDK/Sources/Component/WXLoadingIndicator.m

@@ -23,6 +23,7 @@
 @implementation WXLoadingIndicator {
     UIActivityIndicatorView *_indicator;
     UIColor *_indicatorColor;
+    id _animating;
 }
 
 - (instancetype)initWithRef:(NSString *)ref type:(NSString *)type styles:(NSDictionary *)styles attributes:(NSDictionary *)attributes events:(NSArray *)events weexInstance:(WXSDKInstance *)weexInstance {
@@ -31,6 +32,9 @@ - (instancetype)initWithRef:(NSString *)ref type:(NSString *)type styles:(NSDict
         if (styles[@"color"]) {
             _indicatorColor = [WXConvert UIColor:styles[@"color"]];
         }
+        if (attributes[@"animating"]) {
+            _animating = attributes[@"animating"];
+        }
     }
     return self;
 }
@@ -45,6 +49,13 @@ - (void)viewDidLoad {
     if (_indicatorColor) {
         _indicator.color = _indicatorColor;
     }
+    if (_animating) {
+        if ([WXConvert BOOL:_animating]) {
+            [_indicator startAnimating];
+        } else {
+            [_indicator stopAnimating];
+        }
+    }
 }
 
 - (void)updateStyles:(NSDictionary *)styles {
@@ -54,6 +65,19 @@ - (void)updateStyles:(NSDictionary *)styles {
     }
 }
 
+- (void)updateAttributes:(NSDictionary *)attributes
+{
+    if (attributes[@"animating"]) {
+        _animating = attributes[@"animating"];
+        
+        if ([WXConvert BOOL:_animating]) {
+            [_indicator startAnimating];
+        } else {
+            [_indicator stopAnimating];
+        }
+    }
+}
+
 #pragma mark - lifeCircle
 
 - (void)viewWillUnload {