Added support for expandable named placeholders.

• %(NAME)*s, %(NAME)*q expand named values as array items.
• %(NAME)**s, %(NAME)**q expand named values as key value pairs.
• Pass values for named array items using NAME=ITEM_N syntax.
• Pass values for named key value pairs using NAME=KEY_N NAME=VALUE_N syntax.

Also, changed the syntax for optional placeholder from $?(...) to $(...)?.