Allow access to Letter Element.

Author: kevzlou7979

gwt-material/src/main/java/gwt/material/design/client/base/mixin/LetterMixin.java

@@ -54,4 +54,8 @@ public void setLetterColor(Color letterColor) {
     public void setLetterBackgroundColor(Color letterBackgroundColor) {
         span.setBackgroundColor(letterBackgroundColor);
     }
+
+    public Span getSpan() {
+        return span;
+    }
 }

gwt-material/src/main/java/gwt/material/design/client/ui/MaterialChip.java

@@ -204,4 +204,8 @@ public void setLetterColor(Color letterColor) {
     public void setLetterBackgroundColor(Color letterBackgroundColor) {
         letterMixin.setLetterBackgroundColor(letterBackgroundColor);
     }
+
+    public LetterMixin<MaterialChip> getLetterMixin() {
+        return letterMixin;
+    }
 }