Fix bug that "input" cannot be recognized as an option

[yzyzsun]

I found symbol :input can be recognized as an option of Kramdown::Document.new, but string "input" cannot. Then I found out that it is because @ is omitted in initialize method.

This can also fix that input option does not work in Jekyll's _config.yml since Jekyll passes input as a string.

[gettalong]

Thanks for the bug fix!