feat(loader): support options.productionMode

[pi0]

With webpack 4, this.minimize value is undefined. This PR allows enabling minimize with loader options.

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A side note from here:

Guideline: Loaders should receive all options via this.query. They should not use other ways to receive options, i. e. no property in webpack options, no environment variable.

[yyx990803]

I don't think the option should be called minimize since vue-loader itself doesn't perform any minification. productionMode is more explicit.

[pi0]

@yyx990803 done :)