Fix TODO's since they are actually done
Start of tags file handling

Refactor file loading code into one place since it should all work in a similar way.

Added in tag functionality.
- Cluster files are: tag host host host host
- Tag files are: host tag tag tag

Yes to add in remainder of code to load in the files automatically
Bring new 'tag' file into use

/etc/tags and $HOME/.clusterssh/tags is now used

Refacter some code to make it clearer and update docs for new options
Exclude testcover files
Resolving tags by external command

Start coding up resolving tags by an external command instead of a cluster or tag file