Update plugin.py to accept style path from config

[urz9999]

style path is generated from config -> extra -> extra_sass_path like this:
style_path = config["extra"].get("extra_sass_path") if config["extra"].get("extra_sass_path") else "extra_sass"

if not present it will use 'extra_sass' as per documentation. Style path is then passed to the entry_point, thus being available to Sass_Entry.

I wanted to do this because we use mkdocs in a electron + angular project and the relative mkdocs part is inside a folder called docs, so we didn't want to clutter the main dir with extra directories