Add support for specifying parent container

I've updated this PR to include a few tests and squash it down to 1 commit. Closes #14.

There was one hiccup I did not anticipate originally: changing the bar to be absolutely positioned within its container (by default the `body`), prevents it from being visible when the document is scrolled. 

My solution was to set the position to `fixed` only when the parent is the `body`.