Add LOW MAGIC WARNING. This involved creating a channel, modifying init.txt, and the crawl_options.txt docs. Message will trigger when player's magic drops below the percentage set in init.txt (default 10%). Nice for making sure casters don't run around with low magic amounts.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@988 c06c8d41-db1a-0410-9941-cceddc491573

Created by  mtitus_613  on March 6, 2007
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