feat: gem-based themes inheritance

Summary

Support for parent/child themes allowing users to build upon existing themes. See https://github.com/benbalter/jekyll-remote-theme/issues/35#issuecomment-429888249

Motivation

• develop starter themes and allow to build child themes upon
• distribute child themes as gems

Guide-level explanation

• A child theme is a theme than expand an existing gem-based theme

Reference-level explanation

• Only available for gem-based themes
• A child can list a theme dependency in its gemspec (and theme/_config.yml ?)
• Jekyll can apply a parent theme, and a child theme
• Users have only to declare the child-theme in the main _config.yml

/cc @jekyll/themes @jekyll/minima, @mmistakes, @daviddarnes, @nIckgarlis