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.

help_test.go

@@ -71,6 +71,7 @@ func TestHelp(t *testing.T) {
 		Map      map[string]int `help:"A map of strings to ints."`
 		Required bool           `required help:"A required flag."`
 		Sort     bool           `negatable short:"s" help:"Is sortable or not."`
+		Approve  bool           `negatable:"deny" help:"Approve or deny message."`
 
 		One struct {
 			Flag string `help:"Nested flag."`
@@ -118,6 +119,7 @@ Flags:
       --map=KEY=VALUE;...    A map of strings to ints.
       --required             A required flag.
   -s, --[no-]sort            Is sortable or not.
+      --approve/deny         Approve or deny message.
 
 Commands:
   one --required [flags]
@@ -159,6 +161,7 @@ Flags:
       --map=KEY=VALUE;...    A map of strings to ints.
       --required             A required flag.
   -s, --[no-]sort            Is sortable or not.
+      --approve/deny         Approve or deny message.
 
       --flag=STRING          Nested flag under two.
       --required-two