Add the possibility to enable reusing the private key

[MK796]

Hello!

First of all, your tool is just awesome! Please continue your great work!

The tool would, of course only for my point of view, achieve near perfection when there would be the possibility to let the renewal process reuse its key. Via letsencrypt issued wildcards have only 3 months validity. It's not really convenient that I have to generate the TLSA records each renewal again. If the key is reused then this is not needed.

What's your opinion on that?

Cheers,

Marvin