Added Attachment Support for Apprise API

[caronc]

Description:

Related issue (if applicable): n/a

This is the first part of being able to send attachments to the Apprise API Server with respect to this ticket

This PR allows the attachments to be passed along upstream to the Apprise API server.

An add on needs to be created to the Apprise API to temporarily story the attachments to disk so that it can re-transmit them.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@apprise-api-attachments

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" "apprise://your-apprise-api-host/"