Adds css.alwaysExtract config option

[Karolis2011]

I found no logical option how to override those checks. So, I added option that allows to bypass assumptions that everyone uses HMR and doesn't want to extract CSS in to separate file while in development mode.

[LinusBorg]

Provided we want to enable this, instead of introducting another config option I would argue to simply change the current behaviour to respect css.extract during development.

Users can then simply use NODE_ENV in their config file to decide when to set it to true, e.g.:

css.extract: process.env.NODE_ENV === 'production'

or turn the option into a string based one, something like:

css: {
  extract: true | false | 'production'
}

[Karolis2011]

I am concerning that latter suggestion might break backwards compatibility. But it would break in edge cases, like when this value is explicitly set to true. But I like it.

[yyx990803]

Thanks for the PR - I'm not sure if the extra string-based value is necessary, since that is just the default behavior.

[Karolis2011]

If you will point at a way how to enable extract css on development builds, without deleting and readding module sets with webpack chaining, then I will be satisfied and be sure that this is unnecessary. Else I see this necessary.

[yyx990803]

See my commit - I've made it so that setting it to true will enforce extraction even in development.

[Karolis2011]

Oh sorry, didn't see commit 686ec25
I was expecting it to be referenced in your comment. Anyway, thank you and sorry again.