feat: added prop placeholderClass

Author: mfuu

.babelrc

@@ -1,5 +1,6 @@
 {
   "presets": [
+    ["@babel/preset-typescript"],
     ["@babel/preset-env", { "modules": false }]
   ]
 }

.eslintignore

@@ -3,5 +3,3 @@
 
 **/*.svg
 **/*.sh
-
-/docs/*

.eslintrc.js

@@ -0,0 +1,15 @@
+{
+  "semi": true,
+  "useTabs": false,
+  "tabWidth": 2,
+  "endOfLine": "auto",
+  "proseWrap": "preserve",
+  "printWidth": 100,
+  "singleQuote": true,
+  "arrowParens": "always",
+  "trailingComma": "es5",
+  "bracketSpacing": true,
+  "jsxSingleQuote": true,
+  "jsxBracketSameLine": true,
+  "htmlWhitespaceSensitivity": "css"
+}

.prettierignore

@@ -50,9 +50,6 @@ export default {
       list: [{ id: '1', text: 'a' }, { id: '2', text: 'b' }, ...];
     }
   },
-  methods: {
-
-  }
 }
 </script>
 ```
@@ -116,6 +113,7 @@ export default {
 | `ghostClass`       | `String`  | `''`                     | The class of the mask element when dragging                  |
 | `ghostStyle`       | `Object`  | `{}`                     | The style of the mask element when dragging                  |
 | `chosenClass`      | `String`  | `''`                     | Class name for the chosen item                               |
+| `placeholderClass` | `String`  | `''`                     | Class name for the drop placeholder                          |
 
 ## Methods