add CLI option to only show actions taken#258

The current CLI supports both: - `-q` - quiet - `-Q` - really-quiet What I would find really useful is an option to print only actions taken. If no action taken (e.g. link already exists), then output nothing. I'm open to any alternative for the name of the option(s), but I'd suggest something like -a, --actionable show only output when actions are taken e.g. ``` $ dotbot --actionable Creating link ~/bin -> .conf/bin Removing ~/.bashrc Creating link ~/.bashrc -> .dot/bashrc ``` That way, you get the best of `-Q` and yet only show results when anything actually changed. This avoids the, IMO, extraneous lines such as ``` Link exists ~/.conf -> Projects/conf ``` and the boilerplate: ``` All links have been set up All targets have been cleaned ==> All tasks executed successfully ``` I'm not at all sure how this would work with various plug-ins, and obviously there would be some overlap in work related to the idea of a `--dryrun` option (#236)