Retain newlines in usage string when called with `--help` command line argument
Retain newlines in usage string when called with `--help` command line argument

I downloaded the latest version of the `idempotent-bash.sh` and as I tried to verify that it works, I noticed that the usage string output had lost all of the newlines.

This PR simply adds double quotes around the `$IB_USAGE` variable in echo, so that all the newlines would be rendered properly.
metaist/idempotent-bash