Remove the `filter!` macro
Remove the `filter!` macro

Both `filter!` and `take_while!` are one in the same. Because of issue #83,
there isn't a need to for it and might as well just only use `take_while!`
when you want the macro `filter!`.

pwoolcoc/nom,meh/nom,Keruspe/nom,AndyShiue/nom,sourrust/nom,Geal/nom