Added option for defining content id in email attachments

[leonlipe]

Added an option for defining content ids in email attachments, this allow to send an inline image in the message.

I use this plugin for an app, and I though it could be helpful for others.

[antony]

Hi @leonlipe

Thanks for this - I agree this could be very useful for others.

Would you be able to add a test to ensure that this doesn't regress in future? I'm eager to keep the test coverage on this plugin at a high level, again to benefit others.

I'll fix the travis build in the meantime - not sure why it's failing like that.

Cheers,
Antony