Improved content encoding detection.

`get_encodings_from_content()` can now detect HTML in-document content
encoding declarations in the following formats:

 - HTML5
 - HTML4
 - XHTML 1.x served with text/html MIME type
 - XHTML 1.x served as XML

Ref: http://www.w3.org/International/questions/qa-html-encoding-declarations