Inline mode shouldn't add a second WDS client to entry

[alecmev]

This is related to #711. If there is webpack-dev-server/client in the entry of a Webpack config, then WDS should just skip adding it and/or display a warning. The same should probably be done for hot/hotOnly. Sometimes, like when using React Hot Loader, you (at the same time) want to:

1. Use webpack-dev-server CLI
2. Have inline mode enabled (so that it displays a proper "Project is running at")
3. Have an entry with webpack-dev-server/client defined in it explicitly

Edit: Just found #616. My proposed solution is different, but the problem is the same, so, it probably makes sense to merge them. Let me know what's preferred.

[shellscape]

I think it makes sense to continue the discussion on #616. Closing this one for now.