Scroll to top when clicking checkbox of task item from preview

[Sarah-Seo]

Markdown editor consist of CodeMirror and Iframe components. When scroll event occur from one component, it will tell another component.

Clicking checkbox in preview
-> update value of codemirror
-> reset scroll position
-> scroll event is fired from codemirror
-> preview(Iframe) adjust its scroll position to the reset one.

The fix will be shipped to v0.2.