Add functions Get-GitHubIssue and Get-GitHubComment

[ghost]

This PR implements the following functions:

• Get-GitHubIssue: Lists issues for the authenticated user, or from a specified repository, or an individual issue.
• Get-GitHubComment: List comments for a repo, or from an issue, or an individual comment.

Both functions support all documented query parameters.

Both functions support pagination in an ad hoc way, but as you mention in #1, this should be implemented as part of the platform.

Likewise both functions construct query parameters in an ad hoc way, but that too should be part of the platform.

@pcgeek86

[pcgeek86]

LGTM, thank you for the contribution!