theme loader (template inheritance)

First approach: brute force generate tmp-theme starting with the last
* if it's in the first, it's not taken from the second
* if it's not in the first and is in the second, it's taken from the second and not taken from the third(edited)
* the stuff in the initial/first theme is never overridden, it only overrides

Second approach: `add_raw_templates` `build_inheritance_chains` https://github.com/Keats/tera/blob/ea93d650265d074c5e61447128790792d5dda3d4/docs/content/docs/usage.md

Wanted after: each type of template is place in a subdir and using https://github.com/foundpatterns/lighttouch-libs/issues/5 , all within the same virtual theme

(todo: add themes to foundpatterns. get them with peru. port them from pelican to lighttouch. further develop them.)