Transform <space> in cnxml to html

Transforms:

```
<space id="space1" />
<space />
<space count="5" />
<space count="10" effect="underline" />

<space effect="x" />
```

To:

```
<pre class="space" id="space1"> </pre>
<pre class="space"> </pre>
<pre class="space">     </pre>
<pre class="space underline">          </pre>

<div class="not-converted-yet">NOT_CONVERTED_YET: space</div><space
xmlns="http://cnx.rice.edu/cnxml" data-effect="x"/>
```