Added testing spec
Completed Spoiler tag

Fixed error in spec (related to blockquote behavior)
Added embedded cases in spec

Possible annoying case: Spaces are not counted as a valid separator between > and !, meaning that > ! is the same as >!. This could be better handled
Fixed non-spoilers being interpreted as one

Specifically the following case:

`> !`

A bit dirty as a fix (as it affects blockquotes instead of spoilers),
but should still be ok.