[Enhancement] Support for sourcing rules from external locations such as GitHub or S3.

**Is your feature request related to a problem? Please describe.**

As you mentioned in the readme:

```
Sourcing of rules from external locations such as GitHub Release and S3 bucket. If you want this feature natively in Guard, please raise an issue or +1 an existing issue.
```

I didn't see an existing issue yet, so I'd thought I'd raise one...

**Describe the solution you'd like**

The goal is to create a GitHub action for cfn-guard and integrate it into our projects. But we don't want to copy/paste the cfn-guard rules to each project. Therefore it would be nice to import rules from an external location such as GitHub or S3 with a [flag] on the command line.

**Describe alternatives you've considered**

No alternatives are available.

**Additional context**

Add any other context or screenshots about the feature request here.

Start by reviewing the README passage about sourcing rules from GitHub Release and S3, then inspect the cfn-guard command-line interface. Define the requested flag and supported external locations, and verify that projects can import rules without copying them locally.

Written by the indexing model from the issue text.