Image order
When adding images to teh HTML canvas, like the one used in the `ScrollZoomToggler` plugin, sometimes the map has a higher `z-index` and the image is hidden behind the map. It would be nice to either pass a `z-index` or hard-code a high value to ensure the image is on top of the map.

diff --git a/folium/plugins/scroll_zoom_toggler.py b/folium/plugins/scroll_zoom_toggler.py
index e104277..1390e63 100644
--- a/folium/plugins/scroll_zoom_toggler.py
+++ b/folium/plugins/scroll_zoom_toggler.py
@@ -35,9 +35,11 @@ class ScrollZoomToggler(MacroElement):
             {% endmacro %}
 
             {% macro html(this,kwargs) %}
-                <img id="{{this.get_name()}}" alt="scroll"
-                   src="https://cdnjs.cloudflare.com/ajax/libs/ionicons/1.5.2/png/512/arrow-move.png"
-                   onclick="{{this._parent.get_name()}}.toggleScroll()"></img>
+            <img id="{{this.get_name()}}" alt="scroll"
+                 src="https://cdnjs.cloudflare.com/ajax/libs/ionicons/2.0.1/png/512/arrow-move.png"
+                 style="z-index: 999999"       
+                 onclick="{{this._parent.get_name()}}.toggleScroll()">
+            </img>
             {% endmacro %}
 
             {% macro script(this,kwargs) %}
diff --git a/tests/plugins/test_scroll_zoom_toggler.py b/tests/plugins/test_scroll_zoom_toggler.py
index 77feb68..438399d 100644
--- a/tests/plugins/test_scroll_zoom_toggler.py
+++ b/tests/plugins/test_scroll_zoom_toggler.py
@@ -21,7 +21,8 @@ def test_scroll_zoom_toggler():
     # Verify that the div has been created.
     tmpl = Template("""
         <img id="{{this.get_name()}}" alt="scroll"
-        src="https://cdnjs.cloudflare.com/ajax/libs/ionicons/1.5.2/png/512/arrow-move.png"
+        src="https://cdnjs.cloudflare.com/ajax/libs/ionicons/2.0.1/png/512/arrow-move.png"
+        style="z-index: 999999"
         onclick="{{this._parent.get_name()}}.toggleScroll()"></img>
     """)
     assert ''.join(tmpl.render(this=szt).split()) in ''.join(out.split())
['tests/plugins/test_scroll_zoom_toggler.py', 'folium/plugins/scroll_zoom_toggler.py']