Added '<! @endif -->' syntax to allow valid blocks that are hidden by default

HTML blocks that are hidden by default have been introduced by @justmoon in  #15 and released with version 2.0.0.

The un-preprocessed syntax
```HTML
<!-- @if true !>
    <em>hidden</em> <acronym>html</acronym> block
<!-- @endif -->
```
then, though, is not [valid HTML](http://www.whatwg.org/specs/web-apps/current-work/multipage/syntax.html#comments) (nor XHTML/XML):
> **12.1.6 Comments**
> Comments must start with the four character sequence […] (<!--). Following this sequence, the comment may have text, with the additional restriction that the text ***must not […] contain two consecutive U+002D HYPHEN-MINUS characters (--)*** […]. Finally, the comment must be ended by the three character sequence […] (-->).
(Emphasis added.)

So the first two consecutive dashes in `<!-- @endiff -->' are to be avoided.

My patch alters the syntax for any `@endif` in such way, that an initial `<!` *without* consecutive double-dashes is recognized. For simplicity reasons this is *not* context sensitive, so it is not only valid in cases, where the opening `@if…` was followed by a mere `!>` instead of a `-->` (in such a case, this variant should be preferred!), but always (which probably is not-nice™).