Add Random switch to return a random comic

[csandfeld]

Adds a Random parameter to support returning random comic

Description

Added a switch parameter named Random. When called with this switch, a random date between today and 1989-04-16 will be used to return a comic from that date

Related Issue

This adds the feature requested in issue #2

Motivation and Context

Not required at all, but nice to have same functionality as in the XKCD plugin

How Has This Been Tested?

Performed simple tests using PoshBot 0.11.5 running on Windows 10 with PowerShell 5.1, hooked up to a slack channel

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[csandfeld]

Woops ... committed with wrong user and email configured in git. Will close this PR for now and try to fix that before submitting again.