Only match ASCII digits in from_string() method.

[Ovid]

This code currently works:

my $datetime = DateTime::Tiny->from_string('୭୮୯௦-௧௨-௩௪T௫௬:௭௮:௯౦');

But it spits out warnings when you try to print it via as_string() because \d in regexes matches more than just ASCII digits. The following test program shows the problem:

use 5.10.0;
use utf8::all;
my $num_matched = 0;
for ( 1 .. 2**16 ) {
    my $char = chr($_);

    if ( $char =~ /\d/ ) {
        $num_matched++;
        say "$_ => $char";
    }
}
say "$num_matched characters matched";
#350 characters matched

The pull request substitutes [0-9] for \d in the from_string() regex (thus being compatible with older Perls) to fix this issue. There is a test to verify the expected exception.

[coveralls]

Coverage increased (+7.1%) to 93.333% when pulling 14dfc00 on Ovid:master into 36a2439 on dagolden:master.