tpv.github.config: Replace RecursiveConfigParser by DictConfigParser

DictConfigParser holds all config attributes and keeps track of their
respective config files.

Access works like:
config["github"]["user"] or config["github.user"]. iteration works on
section level. Individual config per file can be accessed via
config.by_files()["<filename>"].

New subcommand config for gh:
gh config     -- prints all files
gh config -v  -- prints all files with their config attributes