Update PREPROCESSORS.md to mention the Less plugin + other revisions

[seanCodes]

I recently created an ECM plugin for less, so adding it to the docs here. While I was at it I took the liberty of rewording or adding to some of the approaches, for clarity. I also adding a third approach (inserted as the second approach) where preprocessed modules are used alongside other preprocessed styles.

Let me know if this copy makes sense. Happy to revise or revert anything!

[seanCodes]

@dfreeman Bump, whenever you have time.