fix blank lines for #792 #774

@tommy351 wrapped each line with `<div class="line">` to fix #687, however, there was a problem with codes with blank lines just like #792 and #774.

Adding css style for `.line` may be a solution,  but it's hard to let all the themes change their css files, I think using `span` instead of `div` and adding `\n` for each line can fix this issue without changing the css files, but I am not sure whether it will cause the same problem in #687. (I don't have the same problem in that issue before wrapping the line with `div`).