Add format for stdout

[pscott]

Closes #50

This PR adds a function fn format_for_stdout(&self, format: FormatFunction) that allows users to specify which format they want stdout to use.

One can now write

.log_target(LogTarget::File)
.duplicate_to_stderr(Duplicate::All)
.duplicate_to_stdout(Duplicate::Info)
.format_for_stdout(colored_fmt)

and having stderr without colors, but stdout with colors!