Support disabling index serving

[ForbesLindesay]

What kind of change does this PR introduce?

This is a small feature

Did you add or update the examples/?

no

Summary

If you are doing server side rendering, by sending all HTML requests to a proxy, and your webpack config generates an index.html file, that file will get served instead of your server-side-rendered content for the root of the domain. Passing an index value of false to webpack-dev-middleware prevents it from serving the index file, but it violates the current schema for options passed into webpack dev server.

Does this PR introduce a breaking change?

No, this merely allows an extra value to be passed in for an existing option.

Other information

You can work around this issue by passing in '' which is close enough to false to trigger the desired behaviour in webpack-dev-middleware, but also a string so it passes validation.

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[shellscape]

Thanks for the PR and congrats on your first submission to the project. Unfortunately we're not accepting any new features for the foreseeable future. We do appreciate it though!

[ForbesLindesay]

I'd appreciate an individual response as to why you think this library is better without the attached change. It's difficult not to be offended by a generic response stating that you do not accept any new features, however small, simple or reasonable they are.

What is your suggestion for how this should be handled?