Add a option for Trace logging.

[justinlatimer]

• Configure slog to output trace! level logging.
• Implement a slog::Drain that allows trace! logging to be toggled at runtime.
• Plumb the trace logging toggle up to a new argument --trace, which can be added anywhere in the arguments, even as part of a subcommand.

This probably closes out #59. We'll be able to add trace! logging everywhere and only have it logged only if the --trace flag is passed in.