Support parameter prompting in Custom Actions

Requested by @littlefyr on Twitter, he'd like the ability to have prompts come up for parameters when a Custom Action is invoked, for cases where the parameters may vary per invocation.

He suggested quite a complex format for this including types, option lists and ranges but I think that's a bit overkill, and a simpler name/value association would work fine. So something like "$prompt(<name>)" where the dialog includes '<name>' as the parameter label.