Add an option to show/hide article tools

[ggreer]

I found the floating article tools distracting when reading, so I added an option to hide them. The default behavior is the same: article tools are shown.

I also fixed a bug with defaults not working correctly. _.extend() was stomping over defaults with undefined settings. When the extension was first installed, the defaults were: newspaper style, extra-small text, and extra-narrow margins. Now they're correct: newspaper, large text, and wide margins.

I don't know if you want to merge this or not, but I figured it's worth a shot.

[ggreer]

I should have separated the bug fix and feature into two pull requests, but it's 3:30AM and I'm tired. Zzzzzzzzzz....