Add option processImport set to true by default.

This pull-request add an option to select if we process the import.

For the integration of clean-css in our project, I need to disable the processing of the import statement.

The bad behavior we saw :

We use clean-css to minify a file that is downloaded from another server. So, clean-css doesn't find the css imported with relative path in the server that run the minifier and it just remove the line.

If you want to add this option to the binary, I can patch the pull-request. I don't think that it's necessary since the files will be likely local in this case.