Remove meta from list of block tags...

...in start condition #6 of HTML blocks.  meta tags are used
in some inline contexts (though this isn't valid HTML5), e.g.
in schema.org.  Example:

```
This paragraph is closed right after an opening span:
<span itemprop="contentLocation" itemscope itemtype="https://schema.org/City">
  <meta itemprop="name" content="Springfield">
  <span itemprop="address" itemscope itemtype="https://schema.org/PostalAddress">
    <meta itemprop="addressCountry" content="US">
    <span itemprop="addressLocality">Springfield</span>,
    <span itemprop="addressRegion">Oregon</span>
    <meta itemprop="postalCode" content="97477">
  </span>
</span>
```

Closes #527.

spec.txt

@@ -2029,7 +2029,7 @@ followed by one of the strings (case-insensitive) `address`,
 `footer`, `form`, `frame`, `frameset`,
 `h1`, `h2`, `h3`, `h4`, `h5`, `h6`, `head`, `header`, `hr`,
 `html`, `iframe`, `legend`, `li`, `link`, `main`, `menu`, `menuitem`,
-`meta`, `nav`, `noframes`, `ol`, `optgroup`, `option`, `p`, `param`,
+`nav`, `noframes`, `ol`, `optgroup`, `option`, `p`, `param`,
 `section`, `source`, `summary`, `table`, `tbody`, `td`,
 `tfoot`, `th`, `thead`, `title`, `tr`, `track`, `ul`, followed
 by [whitespace], the end of the line, the string `>`, or