would it be possible to make it register the space too ": " instead of just ":"?
Supporting this would require a significant rewrite of the tab completion feature which currently parses words based on spaces and cursor location.
It would also require changes to how tab completion works, since it currently only tab completes if the cursor is on a word and not on a space. A suffix with a space at the end would result in the cursor being placed on a space, preventing any further tab cycling.
For now, this feature will not support spaces in suffixes but I am going to add this to my to-do list.