(Bug 27539) Allow attributes beginning with a digit in wiktext tag parameters.

(Bug 27539) Allow attributes beginning with a digit in wiktext tag parameters.
Its removal in r70849 breaks ProofreadPage extension.
Restricted r82475 relaxation to just numbers.
Added tests.

This only affects wikitext (tag hooks).
MW_ATTRIBS_REGEX is only used through decodeTagAttributes() calls.
fixTagAttributes() calls decodeTagAttributes(), and would be nastier to
fix, since it is called with HTML parameters (eg. by removeHTMLtags)
but such incorrect parameters grabbed would be removed by validateTagAttributes()