Added support for Gemfile.local

This is a second attempt at PR #3020 .
This PR changes msfenv.rb so that the framework can use a custom Gemfile. Users can create a custom Gemfile.local file and then install it with `bundle install --gemfile Gemfile.local`

Users can create a Gemfile.local to include all groups in the framework's Gemfile along with custom groups.

```ruby
msf_gemfile = File.join(File.dirname(__FILE__), 'Gemfile')
if File.readable?(msf_gemfile)
  instance_eval(File.read(msf_gemfile))
end

group :local do
  gem 'pry'
  gem 'pry-debugger'
  gem 'lab'
end
 ```
Changes to the Gemfile.local file should not change Gemfile or Gemfile.lock

See also:
* PR #2687
* https://dev.metasploit.com/redmine/issues/8701