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.

negatable.go

@@ -0,0 +1,19 @@
+package kong
+
+// negatableDefault is a placeholder value for the Negatable tag to indicate
+// the negated flag is --no-<flag-name>. This is needed as at the time of
+// parsing a tag, the field's flag name is not yet known.
+const negatableDefault = "_"
+
+// negatableFlagName returns the name of the flag for a negatable field, or
+// an empty string if the field is not negatable.
+func negatableFlagName(name, negation string) string {
+	switch negation {
+	case "":
+		return ""
+	case negatableDefault:
+		return "--no-" + name
+	default:
+		return "--" + negation
+	}
+}