Other gems may want to add commands to the nanoc CLI. To do this
they need to run Nanoc::CLI.add_after_setup_proc. Currently, there
is no way to hook into the CLI. This commit adds such a way by
utilizing Bundler. E.g. the gem guard-nanoc could introduce new
commands when added to the Gemfile in this way:

  gem 'nanoc'
  group :nanoc do
    gem 'guard-nanoc'
  end

Presence of any gems in the group :nanoc will cause the library with
the same name as the gem to be required by Bundler during
the initialization of the CLI.

A special group :nanoc is used for two reasons:
  * nanoc may not be the only or primary user of the Gemfile,
    and requiring many unrelated gems will slow down every invocation
    of nanoc with no workaround;
  * existing Gemfiles may include specifications for gems that cannot
    be successfully required, so using the group :default would
    break such existing nanoc installations.

Discover external commands through Bundler