Allow command predicates to manage parameters passed to the commands

When a command is performed with parameters, those are now passed to the
predicate.
The predicate can then return, after the validity boolean, any other
value that will then be passed to the actual command.
If the predicate only returns the validity boolean, the original
parameters are passed through to the actual command.

This allows predicates to manipulate the received parameters, and allows
them to pass the result of an expensive computation to the actual
command, which won't have to recalculate it.

String and table predicates will now also return `core.active_view`.

data/plugins/autocomplete.lua

@@ -621,12 +621,13 @@ end
 -- Commands
 --
 local function predicate()
-  return get_active_view() and #suggestions > 0
+  local active_docview = get_active_view()
+  return active_docview and #suggestions > 0, active_docview
 end
 
 command.add(predicate, {
-  ["autocomplete:complete"] = function()
-    local doc = core.active_view.doc
+  ["autocomplete:complete"] = function(dv)
+    local doc = dv.doc
     local line, col = doc:get_selection()
     local item = suggestions[suggestions_idx]
     local text = item.text