Add $directoryName replacement option.

Added the option --directory-name. It is intended to let you set a
different directory name than the package name. So, for instance, your
top-level directory could get created with the name "foo", even though
your package name is "bar-baz".

--directory-name defaults to being the same as --package-name if not
specified explicity.

This will give you the file name/directory name replacement string
"directory-name" (just like "package-name").

It also gives a templating parameter $directoryName.  It could be used
inside templates, although it is not intended to be used inside
templates.

This is the pull-request for issue https://github.com/fujimura/hi/issues/17.