Make the shared theme something individual themes can easily inherit from

[honzajavorek]

Danube Delta is meant as my shared Pelican setup for several blogs and I want the blogs to have very similar theme, but not completely the same. Sometimes author field won't be relevant (since the author is just one), sometimes I'll want different wording, colors, illustrations, so would be easy to distinguish the blogs from each other.

This basically means the individual Pelican instances need to have a way how to inherit from the shared theme, which is part of Danube Delta. Currently, Pelican supports inheritance just from !simple theme and there's an open issue getpelican/pelican#1092 and Pull Request in progress getpelican/pelican#1327 which try to solve the situation. To make this thing happen, the best I can do is to finish these things right in the Pelican core.