Expose the `syncImport` option

This option allows the user to enable reading files synchronously from disk. This option allows non-unix developers to ensure that their files are built correctly and less won't fail on random undefined errors when the files are locked/cannot be read due to others reading from it at the same time.

We're having huge issues with contrib-less (less, really -- see https://github.com/less/less.js/pull/1390) on Windows and cannot reliably build our LESS files right now.

This has been rather thoroughly tested with the fix in the issue linked above and I haven't seen it failed yet (three hours in on a continuous test).