Fine grained options for wgMathDisableTeXFilter

Add new option to filter only new input.
Now the complete list of possible settings is:
MW_MATH_CHECK_ALWAYS  backwards compatible to false
MW_MATH_CHECK_NEVER   backwards compatible to true
MW_MATH_CHECK_NEW     new option

Change-Id: I455b41c8b8d918f4c34f6c115194d227a8394e0a