FEAT: http-equiv meta tags

[ankostis]

That is a quick'n dirty implementation to output <meta http-equiv="..."> tags.
The above html element gets produces only for those tags in smtgTagsProperties that exist also in the new smtgMetaHttpEquivs GLOBALS array.

• There is no documentation, apart from this description here.
• The existing structure of the projects has not been honored - only the src/OutputPageHtmlTagsInserter.php file was modified.
• It is provided just as a starting base.

[ankostis]

Forgot to mention that i needed this patch to add a Content-Disposition header (and set the downloaded filename of some page).
Which it didn't work on Chrome (only Firefox honored it, but still added an .html extension).
In the end i found that {{DISPLAYTITLE}} achieves the same thing, much better.

Also noting that it this extension doesn't nothing for pages with content-type: text/plain (makes sense).

[kghbln]

The README file needs to be extended, too. I am not sure if the existing integration tests also cover this new parameter.

[kghbln]

I am not sure if we need this here too, since this HTML comment is already added via the following lines

SemanticMetaTags/src/OutputPageHtmlTagsInserter.php

Lines 120 to 123 in f33ad92

	if ( !$this->metaPropertyMarkup ) {
	$comment .= '<!-- Semantic MetaTags -->' . "\n";
	$this->metaPropertyMarkup = true;
	}

[kghbln]

This needs to be adapted. This is the identical comment copied from the following lines:

SemanticMetaTags/src/OutputPageHtmlTagsInserter.php

Lines 135 to 137 in f33ad92

	// If a tag contains a `og:` such as `og:title` it is expected to be a
	// OpenGraph protocol tag along with other prefixes maintained in
	// $GLOBALS['smtgMetaPropertyPrefixes']

[kghbln]

@ankostis Thanks a lot for submitting a pull request. I have commented while doing a first check. It will be great if @JeroenDeDauw could make some comments form a programmers perspective, too.