Attachment Support Added

[caronc]

Description:

Related issue (if applicable): refs #39 and apprise/870

This will allow the Apprise API to accept File Attachments and relay them along to the upstream services that support them.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [ ] Tests added