Add debug logging for converter properties

[richterdavid]

Initial take on addressing issue 123, for feedback.

[richterdavid]

I'd like feedback on the approach, formatting, and whether logged property names should reflect the variable names in the code or the property names in https://wiki.openzim.org/wiki/Metadata. If the latter, I may need some guidance on the mapping. For example, I suspect that main_url maps to Source, but this isn't made clear in the source code.

[rgaudin]

I think metadata should match libzim or use the notation we use in self.creator.config_metadata( (see below in the same file). Keep in mind those can be modified so printing too early might not be the best approach

[benoit74]

Closing it, nothing will have to be done at warc2zim level, see openzim/python-scraperlib#155