Handling for toggling comments

This PR adds a new parameter called `show-comments`, which you can use to toggle on/off display of comments (both XML comments and elements tagged with `data-type="comment"`). Set `show-comments` as follows to turn on display of comments:

````xml
<xsl:param name="show-comments" select="1"/>
````

And as follows to turn off display of comments:

````xml
<xsl:param name="show-comments" select="0"/>
````