Add rule to check the usage of <meta> charset

[alrra]

• Check if the if the meta tag is not included completely within the first 1024 bytes of the document - done by the markdown validator (see: Add rule to check the markup validity of web documents  #28)

• Check if meta tag is not specified as early as possible (before any content that could be controlled by an attacker, such as a <title> element) so to avoid a potential encoding-related security issue in Internet Explorer (Note: this was only an issue with IE6?).

• Check if non-utf-8 encodings are used

• Check if things like utf8 are used (even though this is valid nowadays as the specifications and browsers alias utf8 to utf-8, that wasn't the case in the past).

• Check if the short version is used (i.e.: <meta http-equiv="Content-Type" content="text/html;charset=UTF-8"> => <meta charset="utf-8"> ).

• Other?