fix: Add trim and scoped postcss plugins at the start of plugin list

[znck]

fixes #30

[JounQin]

@znck See #30 (comment)

plugins is always [] at beginning and your fix will not work.

[znck]

Now trim and scoped plugins are added at the start of the plugins list, before any user defined plugins. Just to be sure, I'll add a test so it won't break.

[JounQin]

There is no user defined plugins at all when using vue-loader because .scss files are handled by sass-loader and postcss-loader then it will be transformed into pure .css and no postcss plugins will be passed into compileStyle function.

You can check https://github.com/vuejs/vue-loader/blob/master/lib/loaders/pitcher.js#L92-L105 and https://github.com/vuejs/vue-loader/blob/master/lib/loaders/stylePostLoader.js

[znck]

/cc @yyx990803

[trainiac]

@znck is there something holding up a release of this fix?

[znck]

I am waiting for @yyx990803's take on this.