* fix: Check if negatable duplicates another flag
Add a check for flags with the `negatable` option if the negative flag
conflicts with another tag, such as:
Flag bool `negatable:""`
NoFlag bool
The flag `--no-flag` is ambiguous in this scenario.
* feat: Make negatable flag name customisable
Allow a value on the `negatable` tag to specify a flag name to use for
negation instead of using `--no-<flag-name>` as the flag.
e.g.
Approve bool `default:"true",negatable:"deny"`
This example will allow `--deny` to set the `Approve` field to false.
Cam Hutchison