signatory and signatory-cli usage output improvement

[stephengaudet]

1. the usage output spans 2 lines, one line each for "flags" and "command". the user would benefit from the precise, 1 line description
2. both "flags" and "command" are wrapped in square braces. square braces denote optional cmdline args whereas angle brackets denote required args. so, flags should remain in square braces and command should be in angle brackets

currently:
Usage:
signatory [flags]
signatory [command]

suggested improvement:
Usage:
signatory [flags] <command>