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/detectindent.lua

@@ -299,10 +299,10 @@ local function set_indent_type(doc, type)
   doc.indent_info = cache[doc]
 end
 
-local function set_indent_type_command()
+local function set_indent_type_command(dv)
   core.command_view:enter("Specify indent style for this file", {
     submit = function(value)
-      local doc = core.active_view.doc
+      local doc = dv.doc
       value = value:lower()
       set_indent_type(doc, value == "tabs" and "hard" or "soft")
     end,
@@ -327,11 +327,11 @@ local function set_indent_size(doc, size)
   doc.indent_info = cache[doc]
 end
 
-local function set_indent_size_command()
+local function set_indent_size_command(dv)
   core.command_view:enter("Specify indent size for current file", {
     submit = function(value)
       value = math.floor(tonumber(value))
-      local doc = core.active_view.doc
+      local doc = dv.doc
       set_indent_size(doc, value)
     end,
     validate = function(value)