Add support for GLUE v2

#31 was an attempt to provide support for fixing https://github.com/quattor/template-library-grid/issues/45. Unfortunately it is not really appropriate and there is a simple solution. This is an attempt to implement it. It is backward compatible with pre #31 version. Despite it is an alternative, the code introduced by #31 is currently kept and will be removed when https://github.com/quattor/template-library-grid/issues/45 is fixed. #31 approach is problematic in that it allows potentially abritrary command execution...

Would be good to have it in 14.10 to allow fixing the long standing https://github.com/quattor/template-library-grid/issues/45 issue...
No. For backward compatibility we want to still allow this value even though we ignore it... this has to be handled in the code itself and the hack could be removed in the future (even though I think it is completely harmless).