[Implement sniff ?] Check for ByteOrderMark

[jrfnl]

Decision needed by Theme Review Board:

There is currently no rule to check for a Unicode Byte Order Mark.

As having a BOM in one of the theme files will often give issues when using those files, it might be a good idea to add this as a rule.

Advice: Add rule which disallows having a BOM in theme files.

To do:

• If agreed this should be a rule - add existing Generic.Files.ByteOrderMark sniff to the ruleset.
• Add the rule in the Theme Review handbook to the Requirements page.

[grappler]

Green light to go ahead :
https://make.wordpress.org/themes/2016/07/12/meeting-notes-for-2016-july-12/