This adds an option to direct log output to a file. This feature has been
requested a lot. It's sometimes useful to have this available when running
geth in an environment that doesn't easily allow redirecting the output.

Notably, there is no support for log rotation with this change. The --log.file option
opens the file once on startup and then keeps writing to the file handle. 
This can become an issue when external log rotation tools are involved, so it's
best not to use them with this option for now.