Analogue of '::' GNU getopt extension?

[yvs2014]

Hello,

I can't find out if dart-args has support of 'optional arguments of options'? (If not, it's a nice feature to have with args parser)

It's about funcionality like this (taken from getopt(3)):
Two colons mean an option takes an optional arg; ... This is a GNU extension

[natebosch]

This is not an existing feature. I think we could consider this if the design is simple enough, but would probably hold off on adding it if its overly complex.