Add "Gallery" field to return multiple images in 1 post (#242)

# Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

- Added a `Gallery` option to the return parameters, which will be an array of image links for every image of that post
- The JSON will always show it
- The human readable will only show it if there is more than 1 image

#161

## Type of change

Please delete options that are not relevant.

- [x] New feature (non-breaking change which adds functionality)
- [x] This change requires a documentation update

# How Has This Been Tested?

Please describe the tests that you ran to verify the changes. Provide instructions so we can reproduce. Please also list any relevant details to your test configuration.

- Added new unit tests for these changes

# Checklist

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that provide my fix is effective or that my feature works
- [x] New and existing unit tests pass locally with my changes
- [x] Any dependent changes have been merged and published in downstream modules

Reviewed-on: #242
Reviewed-by: VylpesTester <tester@vylpes.com>
Co-authored-by: Ethan Lane <ethan@vylpes.com>
Co-committed-by: Ethan Lane <ethan@vylpes.com>

docs/cli.md

@@ -26,6 +26,7 @@ $ random-bunny
 Archived = false
 Author = Rabbit_Owner
 Downvotes = 0
+Gallery = https://i.redd.it/sfz0srdrimjc1.png, https://i.redd.it/sfz0srdrimjc1.png
 Hidden = false
 Permalink = /r/Rabbits/comments/1av1rg9/cute_baby_bun/
 Subreddit = Rabbits
@@ -35,6 +36,8 @@ Upvotes = 211
 Url = https://i.redd.it/sfz0srdrimjc1.png
 ```
 
+- The `Gallery` field is only shown when there is more than 1 image returned, which then the `Url` field is the first image of that list.
+
 ## Help
 
 The command also includes a help option in case you are stuck.
@@ -72,9 +75,11 @@ $ random-bunny --json
 
 $ randon-bunny -j
 
-{"Archived":false,"Author":"Rabbit_Owner","Downs":0,"Hidden":false,"Permalink":"/r/Rabbits/comments/1av1rg9/cute_baby_bun/","Subreddit":"Rabbits","SubredditSubscribers":486085,"Title":"Cute baby bun","Ups":210,"Url":"https://i.redd.it/sfz0srdrimjc1.png"}
+{"Archived":false,"Author":"Rabbit_Owner","Downs":0,"Hidden":false,"Permalink":"/r/Rabbits/comments/1av1rg9/cute_baby_bun/","Subreddit":"Rabbits","SubredditSubscribers":486085,"Title":"Cute baby bun","Ups":210,"Url":"https://i.redd.it/sfz0srdrimjc1.png","Gallery":["https://i.redd.it/sfz0srdrimjc1.png"]}
 ```
 
+- The `Url` field is the first image of the `Gallery` array
+
 ## Sort
 
 You can also choose the sorting option which reddit will use to return the available posts to randomise from.