highlight: fix problem with linenos and rouge.

Found by @EdMcBane in https://github.com/jekyll/jekyll/pull/3435

The strange regexp we were doing to replace the <pre><code></pre></code>
bits in the Pygments output were wreaking havoc on Rouge output
because Rouge uses <pre>'s to wrap line numbers.

To be consistent, the output from render_* should *not* include
the wrapping <div> and <pre> tags. It should just be what was
inside. We can then wrap it in our own custom tags without using
any regular expressions, as God intended. Death to regular
expressions and HTML manipulation!

lib/jekyll/tags/highlight.rb

@@ -77,8 +77,6 @@ eos
       def render_pygments(code, is_safe)
         Jekyll::External.require_with_graceful_fail('pygments')
 
-        @options[:encoding] = 'utf-8'
-
         highlighted_code = Pygments.highlight(
           code,
           :lexer   => @lang,
@@ -96,26 +94,26 @@ eos
           raise ArgumentError.new("Pygments.rb returned an unacceptable value when attempting to highlight some code.")
         end
 
-        highlighted_code
+        highlighted_code.sub('<div class="highlight"><pre>', '').sub('</pre></div>', '')
       end
 
       def render_rouge(code)
         Jekyll::External.require_with_graceful_fail('rouge')
         formatter = Rouge::Formatters::HTML.new(line_numbers: @options[:linenos], wrap: false)
         lexer = Rouge::Lexer.find_fancy(@lang, code) || Rouge::Lexers::PlainText
-        code = formatter.format(lexer.lex(code))
-        "<div class=\"highlight\"><pre>#{code}</pre></div>"
+        formatter.format(lexer.lex(code))
       end
 
       def render_codehighlighter(code)
-        "<div class=\"highlight\"><pre>#{h(code).strip}</pre></div>"
+        h(code).strip
       end
 
       def add_code_tag(code)
-        # Add nested <code> tags to code blocks
-        code = code.sub(/<pre>\n*/,'<pre><code class="language-' + @lang.to_s.gsub("+", "-") + '" data-lang="' + @lang.to_s + '">')
-        code = code.sub(/\n*<\/pre>/,"</code></pre>")
-        code.strip
+        code_attributes = [
+          "class=\"language-#{@lang.to_s.gsub('+', '-')}\"",
+          "data-lang=\"#{@lang.to_s}\""
+        ].join(" ")
+        "<div class=\"highlight\"><pre><code #{code_attributes}>#{code.chomp}</code></pre></div>"
       end
 
     end