Accept more than one --

[sourcefrog]

In building a Cargo subcommand I want to accept commands that look like

cargo mutants --mutants-opt -- --cargo-test-opt -- --test-target-opt

That is to say there will be two -- in the argv seen by argh.

I know this looks a bit weird but I think it is consistent with the general GNU/Unix approach that -- stops special interpretation of options following it and treats them all as arguments.

Unfortunately Argh currently seems to complain

; cargo mutants -- -- --test-threads 1
Unrecognized argument: --

Related: #78, sourcefrog/cargo-mutants#15

[sourcefrog]

Actually this was my mistake, it works fine.