Conditional comments at email button mixin

It seems that Jade no longer parses comment blocks.
As a result, the conditional comment at the button mixin got rendered like:
```html
<!--[if mso]v:roundrect(xmlns:v='urn:schemas-microsoft-com:vml', xmlns:w='urn:schemas-microsoft-com:office:word', href=args.url, style='height: 40px; v-text-anchor: middle; width: 100%;', arcsize=Math.ceil((styles.border_radius / styles.height) * 100) + '%', strokecolor=styles.border_color, fillcolor=styles.background_color)
 v:fill(type="tile", src=styles.background_image, color=styles.background_color)
 w:anchorlock
 center(style='color: #fff; font-family: inherit; font-size: inherit; font-weight: bold;')Manage
| <![endif]-->
```

The recommended way of writing conditional comments nowadays is:
```html
<!--[if lt IE 8]>
link(rel='stylesheet', href='style-ie8.css')
<![endif]-->
```

See related Jade issue: https://github.com/visionmedia/jade/issues/1345

(There is a discussion about the button mixin at Google Groups: https://groups.google.com/forum/#!msg/keystonejs/tYp45taolxk/w4NLyPJbVRgJ)