Highlighting broken for multi-line constructs

[yyx990803]

When highlighting multi-line constructs, e.g. block comments or ES2015 template strings, highlight.js outputs a single <span> that contains newlines. For example:

var string = `
  I am multiline
`

Is converted to:

<span class="keyword">var</span> string = <span class="string">`
  i am multilne
`</span>

In https:/hexojs/hexo-util/blob/master/lib/highlight.js#L32, hexo wraps each line with an extra <span>, which closes the <span class="string"> early, and causes the rest of that string to be highlighted incorrectly:

<span class="line"><span class="keyword">var</span> string = <span class="string">`</span><br>
<span class="line">  i am multilne</span><br>
<span class="line">`</span></span><br>

In addition, it also creates a .line span that is in fact multiple lines high.

I personally use a custom theme that does not make use of the .line class at all, so I simply removed that wrapping span and everything works properly. However it seems many existing themes rely on the .line class for styling :(