Use Buffer internally instead of string

Currently htmlparser2 uses string as the Tokenizer buffer (`this._buffer`). By default it uses UTF-8 encoding when writing. However, when the input page is not in UTF-8 encoding, the characters will be garbled. What I've done here is to use a Buffer instead, thus the encoding will be kept intact. I've also modified the tests so that all of them passes.

But this will be a breaking change because the two callbacks: ontext, oncomment now provide a Buffer as the first argument.

Will you consider to merge this change?