Feture Request: conflictsWith (oppositive than needs)

Let's have
```PHP
$cmd->flag('a')->boolean();
$cmd->flag('b')->boolean()->conflictsWith('c');
$cmd->flag('c')->boolean();
```
to get the following result:
PASS: `(no args)`, `-a`, `-b`, `-c`, `-ab`, `-ac`
FAIL: `-bc`, `-abc`

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