Add shortcut for setting the memory size for VirtualBox VM

plugins/providers/virtualbox/config.rb

@@ -76,6 +76,14 @@ module VagrantPlugins
         @network_adapters[slot] = [type, args]
       end
 
+      # Shortcut for setting memory size for the virtual machine.
+      # Calls #customize internally.
+      #
+      # @param size [Integer, String] the memory size in MB
+      def memory=(size)
+        customize "pre-boot", ["modifyvm", :id, "--memory", size]
+      end
+
       # This is the hook that is called to finalize the object before it
       # is put into use.
       def finalize!

website/docs/source/v2/virtualbox/configuration.html.md

@@ -61,3 +61,12 @@ more than 50% would be used on your own host machine. Some details:
 
 * Multiple `customize` directives can be used. They will be executed in the
   order given.
+
+There is also a shortcut for specifying the memory size (MB) for the virtual
+machine:
+
+```ruby
+config.vm.provider "virtualbox" do |v|
+  v.memory = 1024
+end
+```