Provider logic

[Matars]

Thanks to @Zeviraty and @minhtrancccp for pointing out faults in the provider logic.

@minhtrancccp You also suggested having a token file for managing the credentials, you can do that in the config file. (delete current ~/.config/gitfetch/gitfetch.conf to reset the file)

I will let this PR linger for 1-2 days if anyone wants to test it, I have not been able to recreate any bugs.

In the meantime I am working on better GITFETCH_DEBUG=1 flag and adding it to the --help

Summary

Revamped Provider logic to into a data class that was suggested from this PR #69
This should fix:

• token handling regressions from regressions on gh token handling #68
• should also fix Clearing cache breaks login #70

Changes

• Reworked how providers are handled by using an mew Provider class
• GH_TOKEN unset no longer crashes program if None
• Provider-specific config sections in .conf file ([github], [gitlab], etc.)
• GITLAB_TOKEN support matching GitHub's GH_TOKEN