Use MessagePanelView for the output panel

Hi,

Trying to address the issue of resizing the output panel (e.g., #267, #235), I thought it might be easier to use [`MessagePanelView`](https://github.com/tcarlsen/atom-message-panel), which gives you both resize and minimization for free (and any other benefits that may come up upstream).

I took a stab at implementing this change, which boils down to swapping `ScriptOptionsView` with an implementation that uses `MessagePanelView`. You can see the result on [my fork](https://github.com/ncreep/atom-script/tree/message-panel-view), mostly in the [`script-message-panel-view.coffee`](https://github.com/ncreep/atom-script/blob/531c7deeca4a0fe467d95afa0dbf0f1f4e09c668/lib/script-message-panel-view.coffee) file, which is a drop in replacement for [`script-view.coffee`](https://github.com/rgbkrk/atom-script/blob/ffcf9105514cddff460f1dc6b41e43b7e52085cc/lib/script-view.coffee).
It seems to be fully working. The only thing missing is [the conditional code](https://github.com/rgbkrk/atom-script/blob/ffcf9105514cddff460f1dc6b41e43b7e52085cc/lib/script-view.coffee#L134-L149) that prevents scrolling the view in some cases, since I couldn't figure out the rationale behind it. At the moment, my code just scrolls unconditionally.

If there's interest in this feature, I can turn it into a pull request.