in inline html, attribute values are incorrectly parsed as markdown

Test case:

```js
snarkdown(`<a href="/foo_bar_baz.html">link</a>`)
```

[JSFiddle link](http://jsfiddle.net/x6ub3dfr/)

expected result:

```html
<a href="/foo_bar_baz.html">link</a>
```

actual result:

```html
<a href="/foo<em>bar</em>baz.html">link</a>
```