Add handling of "--"

* Add handling of "**--**" to split script arguments from not to parse arguments
* Add **-h** as also possible help request
* Mark args with **value=...** as flags
* Add optional **ARGUMENTS** variable for additional arguments for usage()

Test script:

    #!/bin/bash
    . optparse.bash
    # Test is used as flag (0|1)
    optparse.define short=t long=test desc='Test run' variable=TEST value=1 default=0
    optparse.define short=n long=nice desc='Nice level' variable=NICE default=10
    # additional arguments for usage()
    ARGUMENTS="-- arg1 arg2 ..."
    source $( optparse.build )
    echo "Test: $TEST"
    echo "Nice: $NICE"
    echo "Remaining arguments: $@"

Results:

    # ./t.sh -h
    usage: ./t.sh [OPTIONS] -- arg1 arg2 ...
    
    OPTIONS:
    
            -t --test:                    Test run [flag]
            -n --nice:                    Nice level [default:10]
            -? -h --help:                 usage
 
    # ./t.sh
    Test: 0
    Nice: 10
    Remaining arguments: 
 
    # ./t.sh --test -n 5
    Test: 1
    Nice: 5
    Remaining arguments:
 
    # ./t.sh --test -n 5 a b c
    Test: 1
    Nice: 5
    Remaining arguments: a b c 
 
    # ./t.sh --test -n 5 -- ~/script --with -x --arguments a b c
    Test: 1
    Nice: 5
    Remaining arguments: /var/www/script --with -x --arguments a b c