Add parser option to verify arity after split

Related to #258.

Currently picocli gives error for this:
```
def cli = new CliBuilder()
cli.a(args: 2, valueSeparator: ',' as char, 'arguments')
def options = cli.parse(['-a', '1,2'])
```
Gives error "error: option '--arg' at index 0 (PARAM) requires at least 2 values, but only 1 were specified: [1,2]"

Add option to split first before verifying arity.