Add `bundle viz --without` to exclude gem groups from resulting graph.

This commit introduces a `--without` flag (similar to the flag on the
`install` subcommand) which will drop the inclusion of these gems from
the graph.

The ability to exclude gem groups from the graph is very useful when
trying to visualize or understand the runtime dependencies of a
particular RubyGem or project, minus any development or test related
libraries.

Previously the process of reproducing this behavior was a manual
process:

* Inspect the project's Gemfile and comment/remove any gems that are not
  runtime dependencies. Typically these gems would be in `:test` or
  `:development` groups.
* Inspect the project's gemspec (if it exists) and comment/remove any
  gems that are not runtime dependencies.

Now it is as simple as:

    bundle viz --without=test development
Merge pull request #3050 from fnichol/viz-without-groups

Add `bundle viz --without` to exclude gem groups from resulting graph.