BugFix: Fix function component not working (#175)

When the provided function takes an optional argument the previous
implementation causes error.

lua/lualine/components/special/functon_component.lua

@@ -1,9 +1,8 @@
 local FunctionComponent = require('lualine.component'):new()
 
-FunctionComponent.new = function(self, options, child)
-  local new_instence = self._parent:new(options, child or FunctionComponent)
-  new_instence.update_status = options[1]
-  return new_instence
+FunctionComponent.update_status = function(self)
+  -- 1st element in options table is the function provided by config
+  return self.options[1]()
 end
 
 return FunctionComponent