Propagate tabindex prop to editor's scroll container

Quill unfortunately doesn't provide an API to set the editor's
tabindex, so we have to override it directly on its dom node.
Also see zenoamaro#232 (comment)

src/component.js

@@ -272,6 +272,7 @@ var QuillComponent = createClass({
 			modules:      this.props.modules,
 			placeholder:  this.props.placeholder,
 			readOnly:     this.props.readOnly,
+			tabIndex:     this.props.tabIndex,
 			theme:        this.props.theme,
 		};
 	},

src/mixin.js

@@ -10,6 +10,9 @@ var QuillMixin = {
 	*/
 	createEditor: function($el, config) {
 		var editor = new Quill($el, config);
+		if (config.tabIndex !== undefined) {
+			this.setEditorTabIndex(editor, config.tabIndex);
+		}
 		this.hookEditor(editor);
 		return editor;
 	},
@@ -83,6 +86,12 @@ var QuillMixin = {
 		editor.setSelection(range);
 	},
 
+	setEditorTabIndex: function(editor, tabIndex) {
+		if (editor.editor && editor.editor.scroll && editor.editor.scroll.domNode) {
+			editor.editor.scroll.domNode.tabIndex = tabIndex;
+		}
+	},
+
 	/*
 	Returns an weaker, unprivileged proxy object that only
 	exposes read-only accessors found on the editor instance,