Signal API Attachment and Group Support Added

[caronc]

Description:

Related issue (if applicable): #223

Added Attachment and Group Support

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
# This way you can just destroy it after when it's all over.
# The below will create a directory called apprise
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@223-signal-enhancements

# Be sure you're running your Signal API Server and query it like so
apprise -b "Test Message" signal://server:port/from_no/to_no

# You can also attach files now too:
apprise -b "Test Message" --attach=/path/to/image.png \
    signal://server:port/from_no/to_no

# Group support added too:
# The following syntax is valid:
#  - @name (translates to `group.name` before post
#  - group.name (no change made)
#  - @group.name (also acceptable, the @ is dropped before post)
apprise -b "Test Message" signal://server:port/from_no/@name1/group.name3/@group.name2

# you can mix and match group's and phone numbers in the Apprise URL

[codecov-commenter]

Codecov Report

Merging #580 (f0cb704) into master (fd0cb3f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #580   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          109       109           
  Lines        14094     14117   +23     
  Branches      2637      2643    +6     
=========================================
+ Hits         14094     14117   +23     

Impacted Files	Coverage Δ
apprise/plugins/NotifySpontit.py	100.00% <ø> (ø)
apprise/plugins/NotifyXML.py	100.00% <ø> (ø)
apprise/plugins/NotifySignalAPI.py	100.00% <100.00%> (ø)

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