Make config file optional

[benmuth]

This fixes #29.

Problems this solves:

• users have control over if/how they want to use the config file
• config file path is customizable
• certain config file options weren't being used at all (for example, the verbose setting was always overwritten by the CLI)

Hopefully this doesn't break any use-cases/workflows. Let me know if there's anything you're skeptical about.

[MicheleCotrufo]

Hey, sorry that it took me so long to look at this. I was looking at your suggested code, and I noticed that you moved the call of the function config.ReadParamsINIfile(config_path) to the main() function inside the main.py file. This should work well when pdf2doi is invoked via command line, but I'm not sure if it would work well when pdf2doi is used as a library by other scripts (for example, from pdf2bib or pdf-renamer. Can you run some test?

[benmuth]

When pdf2doi is used as a library, I think the calling code can just call pdf2doi.config.ReadParamsINIfile(config_path) during initialization (for instance, just before where pdf2bib and pdf-renamer are calling pdf2doi.config.set right now).