Add bundler before/after eval hooks for plugins

[ccutrer]

What was the end-user or developer problem that led to this PR?

My plugin would have a much cleaner implementation if it could hook both before and after the Gemfile is evaluated (the former to easily add additional DSL methods, the latter to do verification that the new methods were properly used).

What is your fix for the problem, implemented in this PR?

Add the hooks and call them from Definition.build (not Dsl#evaluate or Dsl#eval_gemfile, so as not to interfere with the plugin-parsing pass of the Gemfile).

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags