This commit adds the hability to add a special folder through the hook

This commit adds the hability to add a special folder through the hook
'special_mailbox'. As an example I added the spam mailbox. Please,
someone with docs skills at this to the docs 8-)

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@2324 7612ce4b-ef26-0410-bec9-ea0150e637f0