Referenced images inserted directly

[ft-lie]

ref #263
Adding option to insert image directly, and not only via filename.

my $png = make_png();
$worksheet3->insert_image( 'A1',  \$png);

[jmcnamara]

That is something I could (should) add. The Python and C versions of this library support it but no-one asked for it in the Perl version (which predates the others) so I never added it.

I'll close the PR, since this isn't the way I would implement it, but will leave the Feature Request open.

Thanks for the suggestion and PR.