environs/configstore: make Default a variable
Merge pull request #397 from rogpeppe/013-configstore-default

environs/configstore: make Default a variable

It's useful for testing purposes.
WIP Action-get
WIP 2 action-get
WIP action-get tests
WIP: JSON marshaling issues (in context Write?)
Minor tweak
Tests now passing on action-get, no errors in Run.