add LazyConfigValue

exposes container methods on container traits, to be lazily evaluated.

Specifically:

- append, extend, insert on lists
- update on dicts
- update, add on sets
AttributeError check on config no longer works

use new `'Class.trait' in config` check
update bundled profiles to use extend
AttributeError no longer raised on missing config
add LazyConfig.prepend
quotes around "Section.key"
add tests for container extension methods in config
describe config-extend in what's new
Merge pull request #4276 from minrk/config-extend

Support container methods in config