prevent emitting update in middle of applyDelta
prevent emitting update in middle of applyDelta

closes #760

quilljs/quill,willrowe/quill,voxmedia/quill,tim-mc/quill,quilljs/quill,sferoze/quilljs,quilljs/quill,fraywing/quill,fraywing/quill,willrowe/quill,sferoze/quilljs,tim-mc/quill,willrowe/quill,voxmedia/quill,sferoze/quilljs,voxmedia/quill,sferoze/quilljs