Allow async functions for minifyCSS

[chrisspiegl]

I tried using the clean-css option to import / process remote @import statements ({ inline: ['remote'] }).

This option however only works when the clean-css function is called with a callback.

Example based on "How to process remote @imports correctly:

var source = '@import url(http://example.com/path/to/remote/styles);';
new CleanCSS({ inline: ['remote'] }).minify(source, function (error, output) {
  // output.styles
});

Since the html-minifier-terser currently does not support async minifyCSS this is not working.

Async minification for JS was implemented based on this Issue: #49

Maybe CSS is similar to achieve?

[sibiraj-s]

Fixed in v7.0.0-alpha.2