Implement automatic bundle install and caching

* Not yet enabled by default.

action.yml

@@ -13,6 +13,10 @@ inputs:
     description: 'The version of Bundler to install. Either none, 1, 2, latest or Gemfile.lock. The default tries Gemfile.lock and otherwise uses latest.'
     required: false
     default: 'default'
+  bundler-cache:
+    description: 'Run "bundle install", and cache the result automatically. Either true or false.'
+    required: false
+    default: 'false'
   working-directory:
     description: 'The working directory to use for resolving paths for .ruby-version, .tool-versions and Gemfile.lock.'
     required: false