Feature: Handle postcss file extensions if no loader options give

[blake-newman]

= Problem

- vue-loader automatically handles postcss by default, there is many issues
around using custom postcss lang option.
- Using postcss without a lang attribute causes SFC to loose syntax
highlghting

= Solution

- Enable option to alias langs to loader settings.
- specify postcss in lang and maintain default loader configurations

Edit

If no postcss loader is provided in vue loader options then default to
vue-loader handling

This allows you to use vue internal postcss/css handling with known
postcss extensions

fixes: #942
fixes: #800
fixes: #654

[octref]

I'm not against using postcss grammar to highlight css, so <style> by default gets postcss highlight.
However the issue is vscode-css-languageservice does not handle postcss. So autocomplete and other language features won't work.

Vue automatically handles postcss by default

I believe that's a part of Vue's specific webpack configuration in the webpack template. I don't think Vue handles postcss by default (correct me if I'm wrong).

This has been suggested before: #500
My opinion is still the same -- being explicit on lang is better than implicit.
And from Vetur side, it's much easier to build language services according to a spec instead of according to user's ever-changing webpack config.

[blake-newman]

@octref

Yes sorry vue-loader handles postcss by default, using lang="postcss" has known issues as far as i'm aware and i have also not been able to configure it correctly (perhaps doing something wrong).

I was also against #500, this solution does not remove explicitness from the attribute. You still have to provide the lang attribute. It merely aliases a lang to another. <style lang="postcss"> with the alias set to { postcss: 'css' } will run lang postcss with vue internals.

I do believe if possible moving into the Vetur will be best approach, as far as i could work out it was not a trivial thing to implement.

On further thoughts, we should internally handle aliasing postcss to use vue-loaders internals unless overridden in loader configuration.

[blake-newman]

I don't think adding a public API is good, as it could lead to bad implementations now thinking about it.