Support encoding schemes (scheme attribute)

Just throwing this out there without having put much thought into it yet.

It would be good to be able to support the scheme attribute to specify encoding schemes.
The problem I suppose is that if multiple schemes could be used how do you know what has been used.

Maybe each tag definition could have a list of available schemes that can be selected via a select box. Or it can just be left blank.
The definitation could also possibly have a default scheme defined, although that might be less often used.

So for example a date tag could have the allowed schemes of 'ISO 639-2' & 'RFC 1766'.

So then users who care to can select the scheme for their tag and it gets added to the <meta> tag.