feat: add editorWillMount event (#49)

* Update MonacoEditor.js

An event named "beforeCreateEditor" has been added to allow developers to process the Monaco object before creating the editor,Some plug-ins need to use the Monaco object, such as "emmet-monaco-es"

* Update README.md

add new event beforeCreateEditor

* Rename the event

* oops

* Update README.md

README.md

@@ -141,12 +141,19 @@ window.MonacoEnvironment = {
 
 ### Component Events
 
+#### `editorWillMount`
+
+- Params:
+  - `monaco`: [`monaco module`](https://microsoft.github.io/monaco-editor/api/index.html)
+
+Called before mounting the editor.
+
 #### `editorDidMount`
 
 - Params:
   - `editor`: [`IStandaloneCodeEditor`](https://microsoft.github.io/monaco-editor/api/interfaces/monaco.editor.istandalonecodeeditor.html) for normal editor, [`IStandaloneDiffEditor`](https://microsoft.github.io/monaco-editor/api/interfaces/monaco.editor.istandalonediffeditor.html) for diff editor.
 
-Editor is created.
+Called when the editor is mounted.
 
 #### `change`
 

src/MonacoEditor.js

@@ -83,6 +83,8 @@ export default {
 
   methods: {
     initMonaco(monaco) {
+      this.$emit('editorWillMount', this.monaco)
+
       const options = assign(
         {
           value: this.value,