Plugin Developer Tools: Plugin check & fix tool

Automated tool to discover and fix common plugin faults.

https://mclear.co.uk/2020/07/18/suggestions-for-improving-etherpad-plugins/

- [x] Adds CI and updates existing travis configs.
- [x] Adds a LICENSE
- [x] Adds a .gitignore
- [x] Adds a README and checks it includes a reference to the license
- [x] Recommends translations
- [x] Checks for files that shouldn't exist and removes them (.ep_initialized, npm-debug.log)

still to do in the distant future depending on usage.
- [ ] Check packages.json includes link to github repo
- [ ] Checks Etherpad is referred to as Etherpad Lite
- [ ] Checks README includes animated gif.

bin/plugins/lib/LICENSE.md

@@ -0,0 +1,13 @@
+Copyright [yyyy] [name of copyright owner]
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.