[WEEX-657][iOS] Add a feature to control the offset of list attach to the bottom` (#1813)

I have add a '_isContentAttachBottom' attribute in WXListComponent to
 controll whether open this feature. The Weex Component can pass the
‘_isContentAttachBottom’ variable from js to native by 'initWithRef'
 and 'updateAttributes' function. The '_isContentAttachBottom' variable
will be analysed to a bool type and cached in WXTableView object.
 When the WXTableView object is called the 'setFrame' function,if the
'_isContentAttachBottom' variable is true , then the 'contentOffset'
 function of WXTableView object will be triggered to keep the content offset
at the bottom.When the WXTableView object is running the transition
animation, the 'setFrame' will be call every 16 msec . So when the animation
is excuting，the content offset of list will be alaways attached to the bottom .

feature: 657

Author: blackteachinese

ios/sdk/WeexSDK/Sources/Component/WXListComponent.mm

@@ -32,6 +32,14 @@
 #import "WXRefreshComponent.h"
 #import "WXLoadingComponent.h"
 
+@interface WXListComponent () <UITableViewDataSource, UITableViewDelegate, WXCellRenderDelegate, WXHeaderRenderDelegate>
+
+@property (nonatomic, assign) NSUInteger currentTopVisibleSection;
+// Set whether the content offset position all the way to the bottom
+@property (assign, nonatomic) BOOL contentAttachBottom;
+
+@end
+
 @interface WXTableView : UITableView
 
 @end
@@ -79,6 +87,18 @@ - (void)setContentOffset:(CGPoint)contentOffset
     [super setContentOffset:contentOffset];
 }
 
+- (void)setFrame:(CGRect)frame {
+    [super setFrame:frame];
+    if (![self.wx_component isKindOfClass:[WXListComponent class]]) return;
+    BOOL contentAttachBottom = [(WXListComponent *)self.wx_component contentAttachBottom];
+    if (contentAttachBottom) {
+        CGFloat offsetHeight = self.contentSize.height - CGRectGetHeight(self.bounds);
+        if (offsetHeight >= 0) {
+            [self setContentOffset:CGPointMake(0, offsetHeight) animated:NO];
+        }
+    }
+}
+
 @end
 
 // WXText is a non-public is not permitted
@@ -114,12 +134,6 @@ - (NSString *)description
 }
 @end
 
-@interface WXListComponent () <UITableViewDataSource, UITableViewDelegate, WXCellRenderDelegate, WXHeaderRenderDelegate>
-
-@property (nonatomic, assign) NSUInteger currentTopVisibleSection;
-
-@end
-
 @implementation WXListComponent
 {
     __weak UITableView * _tableView;
@@ -144,6 +158,7 @@ - (instancetype)initWithRef:(NSString *)ref type:(NSString *)type styles:(NSDict
         _completedSections = [NSMutableArray array];
         _reloadInterval = attributes[@"reloadInterval"] ? [WXConvert CGFloat:attributes[@"reloadInterval"]]/1000 : 0;
         _updataType = [WXConvert NSString:attributes[@"updataType"]]?:@"insert";
+        _contentAttachBottom = [WXConvert BOOL:attributes[@"contentAttachBottom"]];
         [self fixFlicker];
     }
 
@@ -198,6 +213,9 @@ - (void)updateAttributes:(NSDictionary *)attributes
     if (attributes[@"updataType"]) {
         _updataType = [WXConvert NSString:attributes[@"updataType"]];
     }
+    if (attributes[@"contentAttachBottom"]) {
+        _contentAttachBottom = [WXConvert BOOL:attributes[@"contentAttachBottom"]];
+    }
 }
 
 - (void)setContentSize:(CGSize)contentSize