TODO
- keep track of previous command outputs for last command, and any previous command with target list and command line
- calling commands and alieses from command line
- tagging and filtering commands (so oupgrade gets a tag, and we can filter them)
- saving the last host set in a .file
- allow server groups
- allow grepping of output from a command, and then populating the target list from matchintg on it
- add 'watch' to run a command repeatedly until it succeeds (interruptable)
- add the number of servers for each group of services too
- notice when you don't sudo in front of command and it errors and ask you if you meant to sudo
- autocomplete for environment variable names (requires heirarchical completer)
- implement target aliases (-filteralias) which gives a label to an argument to -hosts
- ad-hoc host groupings with assigned names, and a host grouping stack
- Assign each output group to a number that can be easily selected with a -target etc. But also allow matching
  against those groups. Output groupings survive until the next command.
- allow scripts that use -safe to prompt for safety / restore safety after running
- Add variables that can be set and passed to commands
- implement various commented commands in the command list
- implement interactive alias system
- Catch more exceptions in fabtools, and also add retries
- Make the runner aware of multiple commands so that it can combine outputs and make 'overall success' or 'overall failure'
- make C-c break the connections not the program
- make a /summary command that *only* outputs the summary of a remote command, not the stdout/stderr