Fix retain inversion across multiple ops

[alecgibson]

Consider this base document:

new Delta()
  .insert('123')
  .insert('4', { bold: true });

And this delta:

new Delta().retain(4, { italic: true });

This simply applies italic to the whole document. We'd expect the inverse to remove italic:

new Delta().retain(4, { italic: null });

However, we currently get an inverse op with the wrong retain length:

new Delta().retain(8, { italic: null });

This is because when we parse multiple ops in the inversion, we use the original op's retain value (in this case 4 gets used twice), instead of the length of the individual ops (3 and then 1).

This change updates the inversion to use the correct length for the inverted retain.

[coveralls]

Coverage remained the same at 99.029% when pulling 9ffcab4c4429900c030081b5a912c215c5d0702c on alecgibson:fix-invert into 967fe84dfd6634c02be9bb3c46642f1e59384657 on quilljs:master.

[jhchen]

Great PR thanks!