[markdown mode] Support leading whitespace in fencedCodeBlocks mode name

GFM actually supports whitespace between ``` and the mode for syntax-highlight.
That is, this
```markdown
``` javascript
if (foo > 42) return;
``​​`
```

will result in this
``` javascript
if (foo > 42) return;
```

while CodeMirror doesn't syntax-highlight the code block.