Trillian log server and signer config file does not allow comments

It would be great to allow comments in the config file for `trillian_log_server` and `trillian_log_signer` via the `--config` option. The library being used interprets everything as a token, even commented lines, which then fails to fully parse the actual config file options.

https://github.com/google/trillian/blob/ac281dc7977d2a28952afb1f958917e15a1f04cf/cmd/flags.go#L27