error with positional parameter value starting with -

given
```
		ARGPARSE_POSITIONAL(STRING, "infile", &inpath, "name of input file"),
		ARGPARSE_POSITIONAL(STRING, "outfile", &outpath, "name of output file (or -)")
```

running `./prg in.dat -` will fail.

fix: in argparse_parse_args:
```
        } else if (starts_with(arg_string, "--")) { // "-" -> "--"
```
