Add Vue.extend support, add missing info about Vue.mixin check in readme

Author: michalsnik

Diff for: README.md

@@ -48,6 +48,8 @@ module.exports = {
 All component-related rules are being applied to code that passes any of the following checks:
 
 * `Vue.component()` expression
+* `Vue.extend()` expression
+* `Vue.mixin()` expression
 * `export default {}` in `.vue` or `.jsx` file
 
 If you however want to take advantage of our rules in any of your custom objects that are Vue components, you might need to use special comment `// @vue/component` that marks object in the next line as a Vue component in any file, e.g.:

Diff for: lib/utils/index.js

@@ -384,7 +384,7 @@ module.exports = {
       callee.object.type === 'Identifier' &&
       callee.object.name === 'Vue' &&
       callee.property.type === 'Identifier' &&
-      (callee.property.name === 'component' || callee.property.name === 'mixin') &&
+      ['component', 'mixin', 'extend'].indexOf(callee.property.name) > -1 &&
       node.arguments.length &&
       node.arguments.slice(-1)[0].type === 'ObjectExpression'
 

Diff for: tests/lib/utils/vue-component.js

@@ -126,6 +126,12 @@ function invalidTests (ext) {
       parserOptions,
       errors: [makeError(1)]
     },
+    {
+      filename: `test.${ext}`,
+      code: `Vue.extend({})`,
+      parserOptions,
+      errors: [makeError(1)]
+    },
     {
       filename: `test.${ext}`,
       code: `