Add Option to log to file

[nlowe]

cview clears the screen when it exits. Can we turn this off so we can still read panics if they happen? Additionally, we should add an option to tee the logrus output to a file to make reporting bugs easier.

[nlowe]

According to gdamore/tcell#262 this may not be possible for all terminals (and doesn't look to be implemented for any platforms yet). We will have to figure something else out for panics.

[tslocum]

Hey @nlowe, you should be able to read panic messages when using cview because of the following in application.go:

// We catch panics to clean up because they mess up the terminal.
defer func() {
	if p := recover(); p != nil {
		if screen != nil {
			screen.Fini()
		}
		panic(p)
	}
}()