add middleware option

[deju]

Anyone just want to serve index.html after webpack build, the router was management by front end developer. So I add middleware option, and write returnIndex.js in middleware directory.

[BigBlueHat]

@deju I love the idea of this PR! Thank you.

However, we're needing to refactor http-server soon to remove our dependency on flatiron/union, so I'd rather not introduce a feature which uniquely depends on it--see #483.

Once #483 is solved, this seems like a great feature to consider for easier extensibility.

Thoughts welcome!

[thornjad]

While I think this is a neat feature, I'm not so sure this is within the scope of http-server, which is a static content server. Would this be better suited as a companion package which depends on http-server, or a middleware-focused library like express?

[thornjad]

Closing due to the lack of activity since opening, please discuss and reopen if this feature is still wanted