Review block expression opening and closing tag colours

[daaain]

As per @tyleha's comment on #95, now HTML tags have the same colour as block expression opening and closing tags, which isn't great.

The distinction of block tags is useful I think (as mentioned in #94), but we should pick a different scope.

There's a list of scopes here: http://www.sublimetext.com/docs/3/scope_naming.html#color_schemes

But I couldn't find a nice way to see these visualised, so it's a bit of a trial and error to see what colour is which. Also, the scope should be semantically correct.