Make generator expression syntax accept nested expressions

[poke]

Expand the generator expression syntax, used in both the generator and builder properties, to accept nested expressions within or and replace expressions. The following use cases are of interest:

• {or:{url:1},2} – to use url-matches instead of the normal finder-matches.
• {replace:{url:1},regexp,replacement} – similar, to run a replacement on an url-match.
• {or:1,{replace:2,regexp,replacement}} – to run a replacement on the match first.
• {replace:{or:1,2},regexp,{3}} – and other really crazy combinations.

To support this, the expression functions would implicitely translate single numbers in parameters automatically to {num:N} (or {N}), providing full backwards compatibility.

Alternative would be to use a sandbox for this, but I feel that adds a lot more complexity than necessary.