`--local` option for `bundle gem` command

The `bundle gem` command helps a lot when splitting application code out into gems. But these gems are usually only used locally (i.e. with `gem 'gemname', path: 'gem-subdirectory/` in the `Gemfile` of the parent app) and so they need less code generated. This PR adds a `--local` option to `bundle gem` that will omit the generated code that applies only to gems that will be published to a remote gem server.

I'll add some tests if this functionality is mergable. I just wanted to make sure everyone was okay with the idea first...