Invoking a dry-run with `autostow.sh -n` will simply print out the
directories it would operate on, and those it would ignore and exit.
Useful for debugging, not much more.
With bootstrap files no longer linked to config directory, we can have
an easier directory structure with less nesting for them. Install.sh
needs to use the new structure when calling its bootstrap scripts.
By default autostow.sh ignores assets/ and bootstrap/ directories. This
behavior can be changed through the `AUTOSTOW_IGNORED_DIRS` environment
variable. It should contain the name of the directories to ignore,
separated by a `:`.
