Expose LaTeXML Settings

Make LaTeXML setting configurable with the new global
variable $wgDefaultLaTeXMLSetting.

PS: This variable can be specified as an array or a
string. If specified as an array, the
 array('a'=>'b','c'=>array('e','f'))
would be transformed to the equivalent setting
 a=b&c=e&c=f
,which is the input format for the LaTeXML daemon.

Change-Id: I2869df27cee83b426c6eb2312306fac9d6203ef2