Create a feeder module

[Glutexo]

Maybe the whole architecture can be different. The current idea of having three main components directly in Onigumo would result in Onigumo.Parser and Onigumo.Operator only discovering the files for parsing/handling, pasing them to the respective Spider modules. This discovery can be rather handled by a standalone module, directly feeding the Spider modules. The directory listing logic doesn’t need to be duplicated in thin modules doing essentialy nothing.

Downloader is the only component from the main trinity that doesn’t pass its items to a Spider. It still needs to be feeded by the new module though.

This would require to update the diagram in the README. Related to #228 and #233.

What about the name? Ideas: feeder, supplier, discovery. I personally like the -er/-or suffix. Look at queue jargon, what’s being used in the wild.