Handle old config get syntax gracefully

[schlessera]

The wp config get syntax has changed, breaking BC.

Although the command is rather recent and might not have been used much in scripting, we should look into providing graceful handling of the old syntax.

wp config get --global=<global> => wp config get <key> --type=variable
wp config get --constant=<constant> => wp config get <key> --type=constant
wp config get => wp config list