Attributes should be on the Song object, not inside an attrs object

[spilth]

All other implementations of SongPro put the core attributes like title and author on the song object, but the Javascript library currently puts them under an attrs object. Additionally the README shows the non-attrs approach.

My preference is for the Javascript library to be consistent with all other libraries.

Expected:

let song = SongPro.parse(contents);

console.log(song.title);
console.log(song.artist);

Actual:

let song = SongPro.parse(contents);

console.log(song.attrs.title);
console.log(song.attrs.artist);