Implement MarkdownPreviewView::isEqual
Implement MarkdownPreviewView::isEqual

This is expensive to compute for DOM nodes current

Will investigate a better fix in atom/atom, space-pen, or
underscore-plus.

Closes #106

makyo/markdown-preview,grimmer0125/markdown-preview-kramdown,atom/markdown-preview,Galadirith/markdown-preview,ArnaudRinquin/markdown-preview,rugk/markdown-preview,sctlee/markdown-preview,tkssharma/markdown-preview