Add identifier grammar

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

reference/reference.go

@@ -19,6 +19,9 @@
 //	digest-algorithm-separator      := /[+.-_]/
 //	digest-algorithm-component      := /[A-Za-z][A-Za-z0-9]*/
 //	digest-hex                      := /[0-9a-fA-F]{32,}/ ; At least 128 bit digest value
+//
+//      identifier                      := /[a-f0-9]{64}/
+//      short-identifier                := /[a-f0-9]{6,64}/
 package reference
 
 import (