Possibility to use imports processed by other plugins like `postcss-scss`

[pedropalau]

In the following line the @import directive is always treated as a file with the .css extension, but the plugins support now the ".css", ".scss", ".sass" extensions.

https://github.com/mhanberg/jekyll-postcss/blob/acc941e01ab182552fc91ddce93560245f502b04/lib/jekyll/converters/postcss.rb#L56

Should we have the support for other filetypes or maybe allow the use with other plugins like postcss-scss?