Better padding for spoiler text

With no padding currently surrounding [spoiler] text, the white text
gets close enough to the white post background that it's a bit more
difficult to read than it reasonably could be. Pad spoiler text with 2px
to alleviate this difficulty.

Here's a quick example so you can see what I mean: hover over the
spoiler text in [this post](http://meta.discourse.org/t/how-do-you-spoiler-an-image-or-onebox/55)

Signed-off-by: David Celis <me@davidcel.is>