Refactor, css-modules, spring, hot module replacement and fixes

[gauravtiwari]

This one is not merged

[aganov]

@gauravtiwari I'm just wondering should we have to force usage of css-modules. Currently, due to modified asset_pack_path helper there is no option to set modules: false and just use ExtractTextPlugin in dev mode. Maybe making it optional will be a good idea?

[mfazekas]

It seems that this PR enables HMR by default, this can be confusing for example if you have js console open in chrome as it will break in exception

accepting updates to hmr is non trivial thing. You need to deregister old version of your event listeners, and reregister with new version, etc.

I'd liked the old behaviour better where hmr was disabled by default, and whoever needed it could start with ./bin/webpack-dev-server --hot

[matthewd]

This PR covers multiple issues that has been opened lately

Can we please not do that again? #153 effectively became the project's master branch for several weeks, vetoing all other contributions.

[ytbryan]

Once again, thank you for the contribution @gauravtiwari

Perhaps you can split it up into multiple PRs. For example, the one that check yarn and node version could be separate out. :)