Modified markText to allow css

I modified `markText()` to allow css.

Before, you could only style it by using a className
```
cm.markText(startPos, endPos, { className: "someClass" });
```

Now you can add css dynamically.
```
var marker = cm.markText(startPos, endPos, { css: "background-color: blue; color: red" });
marker.clear();
marker = cm.markText(startPos, endPos, { css: "background-color: orange" });
```