Allow per-project configuration

[theKashey]

While being a convention-based solution, one should accept that different projects might require different conventions, no matter why. For example, one can decide to have all derived files in a single folder, not placed next to the sources.

• [ ] provide a way to configure standard constants https://github.com/theKashey/holistic-image/blob/master/src/constants.ts#L20
• [ ] provide a way to completely override naming function