Allow per-file/folder configuration

[theKashey]

Some files might require fine-tune for conversion settings, however, it's not very clear why one might need/want per folder/file configuration for files expected to be generated once.

Probably some quality aspects can be stored in the filename itself

[maraisr]

Maybe just a .holistic.js file in any folder will override config for files in that folder and south. Not liking the idea of filename to encode config — much harder to refactor