GitHubRepositories: Add Dependabot Service Functions (#235)

This adds the following functions to the `GitHubRepositories` module to manage the GitHub Dependabot services.

- `Test-GitHubRepositoryVulnerabilityAlert`
- `Enable-GitHubRepositoryVulnerabilityAlert`
- `Disable-GitHubRepositoryVulnerabilityAlert`
- `Enable-GitHubRepositorySecurityFix`
- `Disable-GitHubRepositorySecurityFix`

There is no `Test-GitHubRepositorySecurityFix` function is there is currently no API for this.

[GitHub Repositories Rest API v3](https://developer.github.com/v3/repos/)

Author: X-Guardian