New `xtc ls` command
New `xtc doctor` command. Currently just fixes symlinks to generator-xtc.

- Symlink checking, generation and removal logic
- utils.fail now accepts %s variables in `message` and extra params similar to console.log().
- added properties to `env` var: projectRoot, xtcRoot
Run a check for the symlink to generator-xtc before any use of the (sub-)generators, except during install when calling the app generator