source_if_exists
and also expecting `file` instead of `source` as env. variable to make that possible without dancing around: ``` #!/bin/bash file=backup && yadm bootstrap; file=install && yadm bootstrap; file=configure &&yadm bootstrap; ```
Add example of bootstrap that can be used to run interactive shell scripts
Fixes #339
This script will, when installed as yadm's bootstrap script, run all executables in $YADM_DIR/bootstrap.d.
