Display help text when passed no options

[Max Ludwig]

Hi there, appreciating the effort you're putting into making this tool 🙏

I have a tiny suggestion though.

Steps to reproduce:

% oha --version
oha 1.1.0
% oha # execute without any options

Actual output:

error: the following required arguments were not provided:
  <URL>

Usage: oha [FLAGS] [OPTIONS] <url>

For more information, try '--help'.

Expected output:

warning: the following required arguments were not provided:
  <URL>

Usage: oha [FLAGS] [OPTIONS] <url>

Arguments:
  <URL>  Target URL.

Options:
  -n <N_REQUESTS>                     Number of requests to run. [default: 200]
  -c <N_CONNECTIONS>                  Number of connections to run concurrently. You may should increase limit to number of open files for larger `-c`. [default: 50]
  -p <N_HTTP2_PARALLEL>               Number of parallel requests to send on HTTP/2. `oha` will run c * p concurrent workers in total. [default: 1]
  -z <DURATION>                       Duration of application to send requests. If duration is specified, n is ignored.
                                      Examples: -z 10s -z 3m.

... truncating here, but I'd expect the full help output ...

References (I'm not the author):
https://clig.dev/#help
https://bvisness.me/you-cant/

[hatoo]

Thanks, your suggestion seems very good!
It will be implemented in #380.