List tag: Add array operations - push, pop, shift, unshift

[eliot-akira]

It would be useful for the List tag to have more array operations.

• push - Append item(s) to end of list
• pop - Remove the last item
• shift - Remove the first item
• unshift - Prepend item(s) to start of list

They could support the short syntax (attribute with no value); and long syntax with attribute name and value, like action=push.