fix(Button): loading button of <a> should vertical align center

Author: youluna

docs/button/demo/component.md

@@ -28,5 +28,6 @@ ReactDOM.render(<Box direction="row" spacing={20} >
     <Button {...props} type="primary">alibaba.com</Button>
     <Button {...props} type="secondary">alibaba.com</Button>
     <Button {...props} type="normal">alibaba.com</Button>
+    <Button {...props} loading>alibaba.com loading</Button>
 </Box>, mountNode);
 ````

src/button/main.scss

@@ -415,6 +415,14 @@
             visibility: visible;
             animation: loadingCircle 2s infinite linear;
         }
+        &:after {
+            content: '';
+            display: inline-block;
+            position: static;
+            height: 100%;
+            width: 0;
+            vertical-align: middle;
+        }
     }
 
     /* custom loading */