Execute bundler in subdirectory

Sometimes we have Git repositories with the app that we want to deploy in a subdirectory. In those cases, we would need to execute `bundle install` in the subdirectory.

It can be enabled by adding a line like this to the `config/deploy.rb` file:

    set :bundle_subdir, 'frontend'

If you accept this proposal, I will also update the documentation. Also, there might be better names for the config value,