preview_styler doesn't handle CSSRule.CHARSET_RULE

Firefox 31.3.0/Plone 4.3.4/c.markitup 0.2: stylesheets can begin with a declaration like

```
@CHARSET "utf-8";
```

but unless they happen to be the first stylesheet, copying the rule in `preview_styles.js` only results in

> HierarchyRequestError: Node cannot be inserted at the specified point in the hierarchy preview_styler.js (line 20)

Workaround fix is

```
} else {
if (r.type != CSSRule.CHARSET_RULE){ // new
ss.insertRule(r.cssText, rule_index++);
} // new
}
```

but that's really just a band-aid.

Start with preview_styles.js and the insertion failure reported at preview_styler.js line 20. Reproduce the case with an @CHARSET declaration in a stylesheet that is not first, then verify the preview handles the rule without the reported HierarchyRequestError.

Written by the indexing model from the issue text.