Externalizing all parameters through .env file

[renzil]

@wmnnd the idea behind this PR is that you don't need to tweak the script file to configure anything such as domains, emails, etc. Just create a .env file and add the required variables and the script will pick it up - for both LetsEncrypt and Nginx. If you don't provide the env file, it functions as before with defaults. Readme docs have also been updated.

[arulrajnet]

@wmnnd Is there any plan to merge this. BTW, thanks for your good work.

[MurzNN]

Thanks, this is a great improvement! Why it isn't merged so long time?

[siriak]

Good job!