Adjust zoom on search result focus

[akstuhl]

What does this PR do?

Adds a configuration setting called search_zoom_pad that decreases the intensity of the zoom-in when a search result is clicked. Clicking a result will still center the viewer on the corresponding line, but will add the specified amount to each end of the bounds that the viewer zooms to.

What issues does it address?

Closes #46

How to test

Enter a search term and click a result in the list. The viewer should center on the corresponding line, but not zoom in so far that the edges of the line's backdrop line up with the edges of the viewer frame. If you change the search_zoom_pad value in _config.yml to a higher value, search result clicks will cause a slighter zoom; if a lower value, a closer zoom.

[jamiefolsom]

@akstuhl looks and works great. 👍