CLI usage message does not mention ability to set --out directory

[cflee]

In #337 the ability to set a directory as --out/-o was implemented. The usage message for the flag at that point was:

Specify path to a file or a directory to save generated objects into. If path is a directory, the objects are stored in that directory. If path is a file, then objects are stored in that single file. File is created if it does not exist.

In #304 the switch of CLI libraries changed the usage message for --out:

Specify a file name to save objects to

This feature of being able to change the output directory seems to be completely undocumented since then.

Since the website docs don't really discuss relatively low level CLI options like these, I think the CLI usage message could be changed to mention this possibility.