support buffered input for hls.parse & buffered output for hls.stringify

[masroorhasan]

HLS.parse takes a text input, however for very large playlists having to load the entire playlist to memory (a string variable) can be very memory intensive. It would be great to support a buffered input so that not the entire playlist has to be loaded to memory all at once. Supporting buffered output for hls.stringify will also allow to have a buffered reader/writer for the entire parse & stringify pipeline.