Log a warning when the HTML reader encounters a nonconformant meta tag

Instead of throwing an exception and skipping the HTML file, log a
warning with a message which makes it more obvious as to what happened.
Merge pull request #1151 from malept/html-reader-non-conforming-meta-tags

Log a warning when the HTML reader encounters a nonconformant meta tag
Added a new ''modified:' metadata tag to be able to specify the
publication time and date and the last modified time and date
independently.

This makes it possible to access the last updated date with {{ article.locale_modified }} in templates.
Additionally, an already delivered feed entry can be corrected by changing the modified date and time, as it is used for atom:update
/ rss pubDate field now.
Merge pull request #1148 from florianjacob/modified_metadata

Added a new ''modified:' metadata tag