moduleapi: Allow soft dependencies via module:depends(mod, true)

CHANGES

@@ -62,6 +62,7 @@ TRUNK
 - Method for retrieving integer settings from config
 - It is now easy for modules to expose a Prosody shell command, by adding a shell-command item
 - Modules can now implement a module.ready method which will be called after server initialization
+- module:depends() now accepts a second parameter 'soft' to enable soft dependencies
 
 ### Configuration
 

core/moduleapi.lua

@@ -136,10 +136,14 @@ function api:require(lib)
 	return f();
 end
 
-function api:depends(name)
+function api:depends(name, soft)
 	local modulemanager = require"prosody.core.modulemanager";
 	if self:get_option_inherited_set("modules_disabled", {}):contains(name) then
-		error("Dependency on disabled module mod_"..name);
+		if not soft then
+			error("Dependency on disabled module mod_"..name);
+		end
+		self:log("debug", "Not loading disabled soft dependency mod_%s", name);
+		return nil, "disabled";
 	end
 	if not self.dependencies then
 		self.dependencies = {};