yin parser: BUGFIX: temporarily allow parsing models with unknown elements
yin parser: BUGFIX: temporarily allow parsing models with unknown elements

Some statements (if-feature) are not supported yet, so fix the parser to
ignore these elements now. Some code was there previously, but it wasn't
sufficient in all cases.

PavolVican/libyang,PavolVican/libyang,lukasmacko/libyang,PavolVican/libyang,lukasmacko/libyang,PavolVican/libyang,PavolVican/libyang