Make valid all relative pathes.

For now Holocron converts all pathes relative to the _config.yaml.
Convert all Config's `dict` items to Config ones.
Rename `path` config section into `paths`.

Also, I should to note that this commit introduce an alias:
config property of Holocron instances now points to config.holocron.
Added `exclude_folders` param to the `iterfiles`.

If `exclude_folders` is set to True the function will skip folder
which not satisfy a pattern.