Adds a -Random switch parameter to return a random comic

[csandfeld]

Adds a -Random switch parameter to Get-Dilbert. When called with -Random it will return a random comic

Description

When called with the -Random parameter, a random date between now and '1989-04-16' is used when fetching the comit

Related Issue

Adds the functionality requested in #2

Motivation and Context

Not required at all, but nice to be able to get a random comic, similar to what can be done with the XKCD plugin

How Has This Been Tested?

Simple usage test performed running PoshBot 0.11.5 in PowerShell 5.1 on Windows 10, 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]

Sorry about the confusion with the other PR. This one should be good. Please approve or reject as you see fit :-)

[devblackops]

Looks good @csandfeld. Thanks!