Add support for dumping ts_proxy log into a file

[nedn]

This would make it easier to investigate issues when ts_proxy misbehaves (see crbug.com/697900)

[pmeenan]

Sorry about the delay. If you pass --logfile "path/to/log.txt" it will record the log messages to the provided file insted of stdout. You'll also want to specify some level of verbosity (-vvvv for all messages)