Hit-a-Hint couldn't find rounded link elements

hit a hint function cannot draw hints for some link elements, such as "Twitter Bootstrap" style button. because... is_viewable function checks document.elementFromPoint(rect.left, rect.top), but it found parent element if link element is rounded.

For example,

<div><a style="display:block; border-radius:4px;" href="foo">foo</a></div>

document.elementFromPoint(rect.left, rect.top); returns div element, it is not a element.

https://github.com/os0x/ChromeKeyconfig/blob/5002a42634547ca75d6c1f989893c8e20c0eb8e7/Keyconfig/js/chrome_keyconfig.js#L581-L582