Add --only CLI option

This came up in a project I'm working on today, and @yzhangmedidata pointed out that it's an outstanding, and apparently popular, [feature request](https://github.com/bundler/bundler-features/issues/59). Here's my code for it--the tests pass, but I'm not sure about what the proper interactions between `--without` and `--only` should be. Making them truly compatible would require more in-depth code changes than I've done here. I'll be happy to address comments.