Runtime exception on invalid (type_ "xyz") attribute

[evancz]

Folks have reported that they can get runtime exceptions if thy add type_ attributes that are not in the spec or are not supported on certain browsers. Reports include:

• select [ type_ "checkbox" ] [] which is not in the spec - #10
• textarea [ type_ "text" ] [] which is not in the spec - https://github.com/elm-lang/html/issues/82
• input [ type_ "date" ] [] has problems in IE - #72

A very thorough suggestion has been made in #87 that would switch these crashes into silent failures. It is unclear if that is better behavior though.