Fix parsing the contents of imported stylesheets

[maxmilton]

Fixes #70 .

Passes a user defined parser to the processor.process options when running postcss for imported files. This allows for any non-standard syntax to be parsed correctly and added to the resulting AST correctly.

The postcss-import plugin does this in a similar way: https://github.com/postcss/postcss-import/blob/master/lib/process-content.js

This also fixes the following warning when using @import:

You did not set any plugins, parser, or stringifier. Right now, PostCSS does nothing. Pick plugins for your case on https://www.postcss.parts/ and use them in postcss.config.js.

[jonathantneal]

This is great, @maxmilton. Thank you

[agconti]

I'm running into this same problem as well in vercel/next.js#10273.

@jonathantneal Do you plan to release this fix?

[maxmilton]

It would be nice to see a release. I've been installing the package from my Github fork for almost a year now 😬