Add positional options support

[marcrittinghaus]

Currently, positional options are not explicitly supported but just remain as unparsed arguments. This commit adds support for positional arguments so that these are treated like other options including parsing and listing in the help output.

Positional arguments can be defined as regular options but with the OPT_POSITIONAL flag:

        OPT_GROUP("Positional options"),
        OPT_INTEGER(0, "posi", &posi, "positional integer", NULL, 0, OPT_POSITIONAL),
        OPT_STRING(0, "poss", &poss, "positional string", NULL, 0, OPT_POSITIONAL),

The long name is used in error messages if the argument cannot be parsed.

This PR addresses issue: #46

[cofyc-bot]

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[cofyc]

@tripulse what do you think?