Support --args.

[01mf02]

This PR implements support for the --args command-line flag.
It collects all arguments after --args except the first occurrence of -- into $ARGS.positional.

Differences with respect to jq:

• jq -nc '$ARGS.positional' --args --foo yields an error, because it considers --foo as a flag, whereas jaq outputs ["--foo"]. However, jq -cn '$ARGS.positional' --args -- --foo yields ["--foo"] for both jq and jaq.
• jq -cn --args '$ARGS.positional' a yields ["a"], whereas jaq yields null.

To invoke jq in a way that is compatible with jaq:

• Always give a filter before --args (or use -f).
• Use --args -- if any argument after --args might start with --.