enhancement: specify cert paths via command line arguments

[vielmetti]

Right now, if I do this:

Eds-MacBook-Pro:~ ed$ sudo go/bin/ssl-tester 
Password:
2019/02/02 01:30:10 open /etc/ssl-tester/fullchain.pem: no such file or directory

and want to make this work, I need to drop files in a certain directory in the file system.

As an enhancement, take a command line argument. It might look something like

ssl-tester --directory ./ssl-tester