Added support for Gemfile.local

Prior to this change, there were two ways to make use of gems that were not included in the Gemfile:
  1) Change the gemfile and rerun bundle install
     - This forces the user to have a Gemfile that is out of sync with the master framework
  2) Start the console with 'ruby -r pry msfconsole.rb'
    - This is hackish and doesn't always work well from outside of the metasploit-framework base folder

This pull request will allow users to make use of certain features present in the framework (e.g. cmd_pry, lab plugin) or make use of other gems for in custom modules, without changing the Gemfile. This PR allows users to create a Gemfile.local file and define custom gems.