Set called console to the one in `bundle config console`

Following @postmodern's #1616 and @indirect's [recommendations](https://github.com/bundler/bundler/pull/1616#issuecomment-22077307) (not to mention a thousand issues later!), 

> IRB alternatives can be selected by setting the config key `console`, for example with `bundle config console pry`.

I am submitting this for code review, as the PR isn't complete.

Comments:
* The alternate console must be in the Gemfile for it to be required. Any way around this? If not, we should add a warning in there.
* In my test (L31), I'm trying to find a way to ensure the console is not installed. Weirdly, the same test twice passes and fails.
* I'm not familiar with the way to convert a file name to a class constant. Maybe there's a standard way (in the project?) already. Thus, Pry and Ripl are hard-coded in and are the only alternates that can be used.
* When invoked, Pry doesn't show line numbers, and an interrupt shuts it down. That's weird.
* Thinking back, I didn't dare add 'pry' as a dev dependency. Should I have?

Improvements welcome.