Configmanager

Pin versions of dependencies

* Adds a mechanism to allow value override via environment variables
* Adds `allow_dynamic_override` for ItemAttribute which means that such predefined attribute would still be overridable with `config.item_attribute` decorated function.
* Documentation update

Methods added:
* Item.get_path()
* Section.get_path()

Something that will probably get moved to hooks:
* Section.item_attribute decorator to register dynamic attributes
* Section.get_item_attribute()

Hide old documentation. Replace with simple Q&A for now.

Adds `Section.get_proxy(*key)` to allow getting hold of a section or item before it actually is added to the section.