add "year" handling to ledger-iterate-regex
fix ledger-iso-date-regex, abusing its name slightly

the year field in dates is sort-of optional, given "year" or "Y"
directives.  Make the date regex understand this.  (This has been lightly
tested with C-c C-a, but the rest of the uses of ledger-iso-date-regex
have only been eyeballed, not properly audited or tested)
Merge pull request #280 from csrhodes/fix-ledger-iterate-regex

Fix ledger iterate regex