added support for koa middleware + some refactoring for the code

[NXTaar]

Added support for koa applications (generator based koa v1.2~). By default, it uses the middleware for Express. If there is the koa flag in the options set to true it will use generator based middleware for koa.
Also did some refactoring for better code readability.

[SpaceK33z]

This would require Node v4+. I'm also wondering that if we want to support Koa, it's maybe better to use a separated file for this. This looks very hacky.

[SpaceK33z]

There are tests now, that also test if it works with express. If you fix the merge conflict, and add tests with koa, I'll take another look.

[SpaceK33z]

@NXTaar, you still interested in fixing this?

[NXTaar]

Yes, I am. At the present moment, I'm just too busy at my work's project, so I'll return to this issue, as soon as I get free.

[grawk]

This PR is interesting to me, because it does attempt to refactor the module to be useful in multiple contexts. I'm currently working on a "construx" module for webpack and finding this module as a great starting point (for reference, construx).

Because the main "construx" module furnishes the middleware, it would call out to my construx-webpack module only to access the memory-fs. Therefore, I need to remove the references to req/res/next from this module's code in order to do that.

I haven't thought through it completely yet, but it seems it should be possible to refactor this module such that the essential part of it (pass a file name, get the bundled file content in the callback) can be pulled up into an exportable function or separate module which this module can depend on.

Anyway, just throwing that out there as food for thought.

[SpaceK33z]

@grawk, I'm open to refactoring this package so that it is easier to add support for other servers (e.g. Koa) or plugins.

I don't have time for this myself currently, so if anyone is willing to take a stab at it: I'll happily review a PR. You could even do this without adding Koa support; that could be done in a second PR.

Note though that there are integration tests now, so it's important to update these, and preferably add more unit tests.

[CrocoDillon]

Hey! I'm interested to see how this PR progresses and am happy to help if needed.

Perhaps a separate file is more interesting if requiring a higher version of Node than previously for connect users is an issue. Something like import webpackDevMiddleware from 'webpack-dev-middleware/koa' could work.

Here's the thing, Koa 2 will be different. So you'd also need import webpackDevMiddleware from 'webpack-dev-middleware/koa2' or something.

[SpaceK33z]

@CrocoDillon, I'm closing this PR since it will need a full rewrite. In #140, the express-specific code is separated from the rest of the middleware. This makes it a lot easier to add support for e.g. Koa. If you want to help, I would really like it if you could help me review that PR. That would definitely speed things up.

[NXTaar]

Hey, PR starter here. 22th i will implement some fixes and improvements according that comments, you guys wrote

[SpaceK33z]

@NXTaar, awesome. I would suggest to base your PR on the changes in #140.

[NXTaar]

@SpaceK33z ok, i'll take a look

[NXTaar]

@SpaceK33z The #140 is good, i suggest to resolve and merge it first, and then i will make a new PR to merge my koa-supporting changes. I want to avoid to doPR's to @grawk's fork. Better we all improve the original :-)