Add dry-run option to generate a list of files that would be removed

[atc0005]

This is already listed at DEBUG log level, so this option would be enabling reporting of eligible files in some sort of format that is easily read and shared (e.g., logged to a ticket of some kind).

Example/concept usage:

sudo ./elbow --paths /var/log/tomcat7/ --recurse --age 120 --generate-report

Regarding an output format, I'm not sure what would work best. Hopefully something concise for the screen and something else to be saved in a file for later review.

[atc0005]

Note: I need to think of the scope as being over a thousand files.

When I ran this recently against a box with a logrotate misconfiguration (thanks Nagios) a little over 1500 files were removed. I would like to see that list, but not directly on the console, at least not one file per line and certainly not twice as the v0.5.0 release is doing.

[atc0005]

I saw an example earlier today of using tabwriter to build a formatted table. I could try the same approach for listing pruned files.

https://golang.org/pkg/text/tabwriter/