Add support for subdomains

[tgarbin]

This will find all . in $CERTBOT_DOMAIN and escape it. e.g foo\.bar\.example\.com

This should resolve issue #1

[angel333]

Hi tgarbin,

I haven't really tried this commit but it looks to me it would break subdomains within zones. See 05fc536 for the solution I went with eventually.

Thank you for your time and effort though!

[tgarbin]

Hi angel333,

I haven't had a chance to properly look over your commit but I appreciate the feedback.

There'll be something there for me to learn. Thanks a lot for providing this repo!