Fix bug where newlines were inserted inside fenced code blocks.

Change firstPass() code that checks for fenced code blocks to check all
of them and properly keep track of lastFencedCodeBlockEnd.
This way, it won't misinterpret the end of a fenced code block as a
beginning of a new one.

markdown.go

@@ -330,9 +330,11 @@ func firstPass(p *parser, input []byte) []byte {
 
 			if p.flags&EXTENSION_FENCED_CODE != 0 {
 				// when last line was none blank and a fenced code block comes after
-				if !lastLineWasBlank && beg >= lastFencedCodeBlockEnd {
+				if beg >= lastFencedCodeBlockEnd {
 					if i := p.fencedCode(&out, append(input[beg:], '\n'), false); i > 0 {
-						out.WriteByte('\n') // need to inject additional linebreak
+						if !lastLineWasBlank {
+							out.WriteByte('\n') // need to inject additional linebreak
+						}
 						lastFencedCodeBlockEnd = beg + i
 					}
 				}