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.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #1444 from davidcelis/spoiler-padding

Better padding for spoiler text