feat(provider): add Mailchimp provider (#1781)

Author: michalbundyra

src/providers/mailchimp.js

@@ -0,0 +1,22 @@
+export default function Mailchimp(options) {
+  return {
+    id: 'mailchimp',
+    name: 'Mailchimp',
+    type: 'oauth',
+    version: '2.0',
+    scope: '',
+    params: { grant_type: 'authorization_code' },
+    accessTokenUrl: 'https://login.mailchimp.com/oauth2/token',
+    authorizationUrl: 'https://login.mailchimp.com/oauth2/authorize?response_type=code',
+    profileUrl: 'https://login.mailchimp.com/oauth2/metadata',
+    profile: (profile) => {
+      return {
+        id: profile.login.login_id,
+        name: profile.accountname,
+        email: profile.login.email,
+        image: null
+      }
+    },
+    ...options
+  }
+}

www/docs/providers/mailchimp.md

@@ -0,0 +1,26 @@
+---
+id: mailchimp
+title: Mailchimp
+---
+
+## Documentation
+
+https://mailchimp.com/developer/marketing/guides/access-user-data-oauth-2/
+
+## Configuration
+
+https://admin.mailchimp.com/account/oauth2/client/
+
+## Example
+
+```js
+import Providers from `next-auth/providers`
+...
+providers: [
+  Providers.Mailchimp({
+    clientId: process.env.MAILCHIMP_CLIENT_ID,
+    clientSecret: process.env.MAILCHIMP_CLIENT_SECRET
+  })
+]
+...
+```