Add support for custom webpack loaders

[gigabo]

From reddit:

How do I use my own webpack loaders? i.e. I'd like to use CSS modules, maybe I have a markdown loader, etc.

We don't have a good answer to that with react-server-cli yet. Maybe via .reactserverrc?

[tyler-dot-earth]

Some freeflow thoughts to help get the conversation started:

• Imagining something like routes config (either CLI or .reactserverrc) where you can point it to a webpack.config file. Then you have the problem of loader ordering - are the configured loaders from the end-user's webpack used BEFORE the ones provided by react-server? Maybe there's concepts like afterBabelLoaderor afterServerLoaders? Seems a bit funky..
• Loaders might cause some issues if they're not isomorphic-friendly, I imagine. (I also haven't tried something that uses window yet but curious to see how react-server handles it)

I'll definitely be poking through the source soon to see if there's anything I can help do to improve this issue, as it's probably the only thing really holding me back from using react-server.

[gigabo]

Yeah it would be tough to automatically merge a whole webpack.config, though maybe webpack-merge could help? Targeted afterBabelLoader-type hooks might be easier to tame, but I wonder how many of them we'd eventually wind up with...

Thanks for looking at this @tsnieman!

[CreepGin]

Agree with the complexity involved with custom webpack configuration. But in the case of CSS Modules, we can safely consider enabling it by default.

[gigabo]

I think #598 took care of this (and #709 added a CLI option for it). 🎉