Remove support for parsed & conditional comments

Parsing non-conditional comments is really confusing and unexpected, we need to stop doing it.

Supporting conditional comments to the extent people want is impossible.  They are largely an anti-pattern as there is almost always a better way to do the same thing.  In the rare case that you do need a conditional comment they can be written inline and will work just fine:

```jade
<!--[if IE lt 9]>
script(src="/lame.js")
<![endif]-->
```

We should just tell people to write their conditional comments like that.