Added core_config_API.php as suggested on the wiki

This provides configget() which takes a config option name (minus the $g part) and returns the value. The given implementation looks for a global variable with $g_ prepended but could easily be altered or added to to look up in a DB or whatever

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1321 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9