Add parser option to verify arity after split

[remkop]

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.

[remkop]

Closing: will not provide explicit API for this, but semantics will be included in #363 ParserSpec.limitSplit.