Allow providing custom PostCSS options

[IlCallo]

• Operating System: Ubuntu 18.04
• Node Version: 12.14.0
• NPM Version: 6.13.4
• webpack Version: 4
• css-minimizer-webpack-plugin Version: 1.2.0

Feature Proposal

Allow specifying custom options for PostCSS process, as https://github.com/NMFR/optimize-css-assets-webpack-plugin does (cssProcessorOptions option).
The options object is defined here and cannot be extended.

Feature Use Case

Into Quasar Framework we use https://github.com/postcss/postcss-safe-parser via the parser PostCSS option.

A possible patch would be to copy/paste the internal minify function into an external provided minify one, with the options we prefer

[alexander-akait]

Yep, sounds good, PR welcome, let's name it as processorOptions

[mutoe]

Are there any updates? If I want to use postcss-safe-parser now, is there any alternative?