NPC updates

This PR is for 2 things:
- use the manaplus [item] system in npc dialogs to give players a thumbnail of the items
- use the manaplus [url] system in npc dialogs to add more links to the wiki

note: for the items, I use [@@**ID**|**getitemname(ID)**@@] instead of [**name**] so that we can change an item name without having to edit all npc files