This change implements a very basic `coco env config` command, that
lets you read, set, or unset configuration values for an environment.
For a single environment, these four usage styles are supported:
# query all values in a given environment <env>:
$ coco env config <env>
# query a single value with key <key> in a given environment <env>:
$ coco env config <env> <key>
# set a single value with key <key> and value <value> in <env>:
$ coco env config <env> <key> <value>
# unset a single value with key <key> in the environment <env>:
$ coco env config <env> <key> --unset
This is a vast subset of pulumi/coconut#113.
d94f9d4768