Allow file based configuration

[liamsi]

Similar to google/trillian#620
And (if not resolved by #578): frontend.go should read the actual PEM file (see #578 (comment))

@gdbelvin proposed using https://github.com/namsral/flag

[gdbelvin]

At the moment, we're not seeing a strong demand for this. We can reopen at a later point if interest comes around.