Let handlers be loaded from a shared library

[FatihBAKIR]

Right now, we need a separate executable for each handler, which isn't strictly necessary for Cspot's operation. Being an executable isn't even part of the API, the writers of handlers is only concerned with their functions. This is the raison d'être of shared libraries.

Loading the handlers from a shared library would allow for having multiple handlers within a single binary, which would be helpful for deployment.