A user might run vicfg and want to reset a line back to the default value
from the value they have periously set. A natural way to do that is to
comment out the line (or delete it). However, what that actually does is
vicfg parses the result and skips over that setting, since it's not
present, and so no change is saved.

It could try to parse commented out lines and detect deleted lines,
but that way lies madness. Also, it's not at all clear what the "default"
should be in response to such an action. The default varies per type of
configuration, and vicfg does't know about defaults.

> [[fixed|done]]; this was a job for Data.Default!  --[[Joey]]

Instead, I think it should detect when a setting provided in the input
version of the file is not present in the output version, and plop the user
back into the editor with an error, telling them that cannot be handled,
and suggesting they instead change the value to the value they now want it
to have.

> Nah, too complicated.
