custom naming schemes

[yoshuawuyts]

@helveticade pointed out that inspecting HTML with just hashes doesn't relay a lot of information per-se; when debugging having something like the file filename / dirname available is neat; perhaps we can expose a hook for people to create their own prefix (in addition to the hash, which guarantees uniqueness). Thoughts?

[ahdinosaur]

for reference, here's how css-modulesify does it, here's how cssify does it, and how webpack does it.