Make negatable flag name customisable (#439)

* 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.

build.go

@@ -315,6 +315,13 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 			}
 			seenFlags["-"+string(tag.Short)] = true
 		}
+		if tag.Negatable != "" {
+			negFlag := negatableFlagName(value.Name, tag.Negatable)
+			if seenFlags[negFlag] {
+				return failField(v, ft, "duplicate negation flag %s", negFlag)
+			}
+			seenFlags[negFlag] = true
+		}
 		flag := &Flag{
 			Value:       value,
 			Aliases:     tag.Aliases,