Add option --mssafe to isodate

MS OSs don't like ":" in filenames (and Samba's scrambling algorithm
makes filenames with it completely unrecognizable), so I added an option
to use a less dangerous character ("-") in the time part. "." would
have been nicer and technically possible but that's used to delimit
extensions, so that might have had surprising consequences, too. Just
omitting the delimiter and squashing HHMMSS together isn't very
readable.

tiny/isodate

@@ -1,2 +1,14 @@
-#!/bin/sh
+#!/usr/bin/perl
-date '+%Y-%m-%dT%H:%M:%S'
+use v5.10;
+use warnings;
+use strict;
+use Getopt::Long;
+use POSIX qw(strftime);
+use Pod::Usage;
+my $mssafe;
+GetOptions(
+    "mssafe" => \$mssafe
+) or pod2usage(1);
+
+my $fmt = $mssafe ? "%Y-%m-%dT%H-%M-%S" : "%Y-%m-%dT%H:%M:%S";
+say strftime($fmt, localtime);