Not recognizes "--" to stop parsing

The **--** stopper is not recognized to stop parsing arguments

Test:

```
#!/bin/bash
. optparse.bash
optparse.define short=t long=test desc='Test run' variable=TEST value=1 default=0
source $( optparse.build )
echo "$@"
```

Result:

```
# ./t.sh -t -- arg1 arg2
Unrecognized long option: --
usage: ./t.sh [OPTIONS]
...
```
