Space control

Hi,

This is a proposal to add space before and after the tag.

```jade
p
  span no space
p
  _span space before
p
  span_ space after
p
  _span_ space before anda after
```

```html
<p><span>no space</span></p><p> <span>space before</span></p><p><span>space after</span> </p><p> <span>space before and after</span> </p>
```