Implement default retro compatibility between v1 and v2

[sylhare]

Since there are some big changes on the v2 in the _config.yml.
I will set up some failsafe so that the older configuration files would still work in most cases.

Once it is done, I'll be releasing the new version of the theme as part of #187

[sylhare]

As a side note.

Retro compatibility has been made possible using:

• Defaults | default: "..." for all text parts
• Logical or site.theme_settings.... for the options so if not in _data it will take the one from the theme_settings.