Support embedded ATTACH property

[calebdw]

Hello!

We would like to be able to send embedded attachments with our Calendar invites to our customers but this is not currently an option.

Per RFC 5545 the iCalendar format does supports embedded attachments, would y'all consider adding this functionality?

Thanks!

Edit: PR #104

[freekmurze]

Feel free to send a PR. If the code is simple, well tested and documented, there's a high change we'll merge it in.