Fix issue with caption on bootstrap 4

Add new prop to check if bootstrap is used
Update component in order to return a custom caption when bootstrap4
is in use

packages/react-bootstrap-table2/src/caption.js

@@ -4,16 +4,22 @@ import PropTypes from 'prop-types';
 
 const Caption = (props) => {
   if (!props.children) return null;
-  return (
-    <caption>{ props.children }</caption>
+
+  const caption = props.bootstrap4 ? (
+    <caption style={ { captionSide: 'top' } }>{props.children}</caption>
+  ) : (
+    <caption>{props.children}</caption>
   );
+
+  return caption;
 };
 
 Caption.propTypes = {
   children: PropTypes.oneOfType([
     PropTypes.node,
     PropTypes.string
-  ])
+  ]),
+  bootstrap4: PropTypes.bool
 };
 
 export default Caption;