Issues/352 blank options

[gschueler]

Enhance option variable replacement to use blank values if a value is present for issue #352

• commands, script args, etc now replace ${option.name} with blank string if the value isn't present
• Default option values are used if an option isn't specified, even if not marked "required"
• Users can specify a blank value, even if an option has a default, as long as it is not "required"
• run and dispatch tools support passing blank arguments (-option1 "")
• Script token expansion of the form @option.value@ will also replace with blank if the value is not available.