@hackmd/codemirror doesn't know of meta-marked

If the first line is `---`, the lines until `---` or `...` are meta info. So codemirror should do a special case.

For example, codemirror/addon/fold/markdown-fold.js can be modified like the following:

```
CodeMirror.registerHelper("fold", "markdown", function(cm, start) {
var maxDepth = 100;
var lastLineNo = cm.lastLine();

var lineZero = cm.getLine(0);
if (lineZero.match(/^-{3}/)) {
var end = 1;
while (end < lastLineNo) {
if (cm.getLine(end).match(/^\.{3}|-{3}/)) break;
++end;
}
if (start.line == 0) {
return {
from: CodeMirror.Pos(0, lineZero.length),
to: CodeMirror.Pos(end, cm.getLine(end).length)
};
}
if (start.line < end) return undefined;
}
```

Well, modifying codemirror/mode/markdown/markdown.js is beyond my skill.