Add trap_item_brand that behaves like stair_item_brand.

Defaults to 'none'. (FR 1793669)

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@2083 c06c8d41-db1a-0410-9941-cceddc491573

Created by  j-p-e-g  on September 13, 2007
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