feat: support multiple option for plugin API.

Since the namesake plugin can only be executed once by default, if your plugin does need to be executed multiple times, you need to set 'multiple' to true explicitly.

Author: ulivz

__mocks__/vuepress-plugin-b.js

@@ -1 +1,3 @@
-module.exports = {}
+module.exports = {
+  multiple: true
+}

packages/@vuepress/core/__test__/plugin-api/Plugin.spec.js

@@ -1,4 +1,5 @@
 jest.mock('vuepress-plugin-a')
+jest.mock('vuepress-plugin-b')
 jest.mock('@org/vuepress-plugin-a')
 
 const Plugin = require('../../lib/plugin-api/index')
@@ -68,4 +69,19 @@ describe('Plugin', () => {
     // using the last one
     expect(plugin.enabledPlugins[0].$$options).toBe(pluginOptions3)
   })
+
+  test('a "multuple" plugin can be applied multuple times.', () => {
+    const pluginOptions1 = { a: 1 }
+    const pluginOptions2 = { b: 1 }
+    const pluginsConfig = [
+      ['b', pluginOptions1],
+      ['b', pluginOptions2]
+    ]
+    const plugin = new Plugin()
+    plugin.useByPluginsConfig(pluginsConfig)
+    expect(plugin.enabledPlugins).toHaveLength(2)
+    // using the last one
+    expect(plugin.enabledPlugins[0].$$options).toBe(pluginOptions1)
+    expect(plugin.enabledPlugins[1].$$options).toBe(pluginOptions2)
+  })
 })

packages/@vuepress/core/lib/plugin-api/index.js

@@ -37,12 +37,13 @@ module.exports = class Plugin {
       return this
     }
     plugin = hydratePlugin(plugin, pluginOptions, this._pluginContext, this)
-    const duplicateIndex = this._pluginQuene.findIndex(({ name }) => name === plugin.name)
-    if (duplicateIndex !== -1) {
-      this._pluginQuene.splice(duplicateIndex, 1)
+    if (plugin.multiple !== true) {
+      const duplicateIndex = this._pluginQuene.findIndex(({ name }) => name === plugin.name)
+      if (duplicateIndex !== -1) {
+        this._pluginQuene.splice(duplicateIndex, 1)
+      }
     }
     this._pluginQuene.push(plugin)
-
     return this
   }