Prioritize HTML comments over regular comments

[winstliu]

--> automatically ends a regular // comment for HTML interoperability. For example,

<script type="text/javascript"><!--
       var hello;
//--></script>

is valid HTML and JavaScript and ends the <script> block correctly.

This does not add support for ES6 HTML-style comments.

Unblocks atom/language-html#90.

[winstliu]

Misunderstood the intended behavior. // automatically ends upon reaching a </script> tag. This makes the problem much harder to solve though.