Addition of a dry run flag

[ThisIsQasim]

@ChrisPates glad to see this project finally getting some much needed attention

Is your feature request related to a problem? Please describe.
This tool has been broken for quite a while and some of us have been running it with our own set of patches. However, now that there is a renewed focus on this, I expect people would want to get back to using the upstream version.

Describe the solution you'd like
In order to minimise the risk when switching sync tool or after version updates a --dry-run option would be very helpful.

[ChrisPates]

This is an excellent feature, its going to take a little house keeping on the code. Currently transactions are executed against the AWS api at various points.

For a clean implementation It would be better to determine all the changes should be made and with the dry run set report them to the logs/console without making them.