Enclose argument names inside [] in --help

[neonichu]

This is more in line with traditional help output by Unix commands.

Before:

$ ./bin/ctf-generator --help
Usage:

    $ ./bin/ctf-generator Space ID Access Token

After:

$ ./bin/ctf-generator --help
Usage:

    $ ./bin/ctf-generator [Space ID] [Access Token]

[kylef]

We discussed this closer to the PR via iMessage and discussed that []means optional inside many different man pages and command line tools and we should do something else here.

Would you mind updating the PR @neonichu?