expose style option for attachment icon

lib/src/message_input.dart

@@ -94,6 +94,7 @@ class MessageInput extends StatefulWidget {
     this.actionsLocation = ActionsLocation.left,
     this.attachmentThumbnailBuilders,
     this.inputTextStyle,
+    this.attachmentIconColor
   }) : super(key: key);
 
   /// Message to edit
@@ -143,6 +144,9 @@ class MessageInput extends StatefulWidget {
   /// `Theme.of(context).textTheme.bodyText2`.
   final TextStyle inputTextStyle;
 
+  /// Color used for attachment icon.
+  final Color attachmentIconColor;
+
   @override
   MessageInputState createState() => MessageInputState();
 
@@ -622,6 +626,7 @@ class MessageInputState extends State<MessageInput> {
         },
         icon: Icon(
           Icons.add_circle_outline,
+          color: widget.attachmentIconColor,
         ),
       ),
     );