Add new cmdlets. Return null instead of throwing error for GetRegistration

[VitaliHarbiankou]

Add new cmdlets:

• Remove-AcmeIdentifier: removes an identifier from vault
• Revoke-AcmeCertificate: revokes an certificate from Let's Encrypt
• Remove-AcmeCertificate: removes an certificate from vault and all the related files

Return null instead of throwing error for GetRegistration

[ebekker]

@VitaliHarbiankou , this looks great, thank you for the additions!

Can you add a few unit tests to cover the changes and additions?

[VitaliHarbiankou]

@ebekker, unfortunately, I can't because I'm not a programmer and didn't write any unit test before. Frankly speaking, it is my first experience with code writing for so long period.