Add "Custom Rules" (a.k.a "specify entries by arbitrary ID") option

[manake]

Feature request:

1. To make this plugin complete it would be necessary to add "CUSTOM RULES" option.
2. What this would accomplish is it would allow specifying individual IDs of posts/pages (something that isn't possible at present).
3. The full interface could taken from Advanced Custom Fields whose "Rules" panel is perfect and has everything covered:

• WooSidebars has 4.1/5 rating because it's incomplete
• Advanced Custom Fields has 4.9+/5 rating, its UI is perfect and covers every usecase