ACTION-193: Known tags must use known tag format

Following Dec. 2, 2015 WG telcon discussion - mandating the use of known table tags requires new conformance test cases. I have added the "MUST" requirement for encoder to use known table tags but I am somewhat reluctant to do the same on the UA side.
- Does it make sense to allow decoders to process an input font even if known tables are encoded using custom table flag with the known tag that follows? Seems like this would be harmless and easy way to preserve backward compatibility.
- Similarly, does it make sense to mandate that only those tables that have a transform flags set be encoded with the known table tag? This would likely be a safer way to deal with the transformed tables but hypothetically would limit our ability to introduce new transforms for new font tables that may be defined in the future.
- Should we simply replicate the MUST use known table tags for the UA without any regard for transform?

Most recent edit makes it a MUST for encoder to use known table tags when processing an input font file but at the same time adds MAY provision for UA to accept WOFF2 files that have known tables encoded with ("custom tag flag" + "table tag") even if the "table tag" is known.