Only download transactions for the last year

Some FIs don't return *any* transactions if you ask them for *all* of
them since the beginning of (UNIX) time...

cmd/ofx/bankdownload.go

@@ -54,7 +54,7 @@ func download() {
 			AcctId:   ofxgo.String(acctId),
 			AcctType: ofxgo.String(acctType),
 		},
-		DtStart: ofxgo.Date(time.Unix(0, 0)),
+		DtStart: ofxgo.Date(time.Now().AddDate(-1, 0, 0)),
 		DtEnd:   ofxgo.Date(time.Now()),
 		Include: true,
 	}