Allow passing multiple Gemfiles to bundle clean

I believe bundle clean should be able to receive multiple Gemfiles to clean gems that are not in any of the associated .lock files.

This feature has already been requested here: rubygems/bundler#2546, but was refused in 2013. I would like this to be considered again and would add a use-case that was not mentioned at the time: Travis-CI 's caching

Some projects, have multiple Gemfile, the deep-cover gem has 7 of them. In deep-cover's case, they are for projects used as fixtures for tests.

Right now, Travis-CI supports caching bundler. However, it will only do it for the main Gemfile (it runs bundle clean before caching). If you want to cache more, you are own your own. In our deep-cover's case, the main Gemfile is very small compared to the small Rails application we use for tests.

The easy way is to just cache everything and never worry about clearing things, but that is quite wasteful (the cache would grow, slowing down the copy process and wasting Travis resources). If bundle clean allowed multiple Gemfiles, it would be trivial for users to set things up to the need of their project, and Travis-CI could add multiple Gemfile caching more easily. (Possibly by just passing in every Gemfile of the project)

Start at the bundle clean command and review the earlier Bundler request referenced in the issue. Determine how multiple Gemfiles and their associated lockfiles should be handled, with completion meaning gems unused by all selected lockfiles are cleaned. No implementation file or test is named, so locate the command's existing tests before changing behavior.