Manually write usage to add --

[asmacdo]

Fixes: #51

Annoyingly this changes the wrapping/indentation, so I added a newline to the start to make it just as easy to read.

Was:

 duct --help      
usage: duct [-h] [-p OUTPUT_PREFIX] [--sample-interval SAMPLE_INTERVAL] [--report-interval REPORT_INTERVAL] [-c {all,none,stdout,stderr}]
            [-o {all,none,stdout,stderr}] [-t {all,system-summary,processes-samples}]
            command [arguments ...]

Now:

duct --help
usage: 
duct [-h] [-p OUTPUT_PREFIX] [--sample-interval SAMPLE_INTERVAL] [--report-interval REPORT_INTERVAL] [-c {all,none,stdout,stderr}] [-o {all,none,stdout,stderr}] [-t {all,system-summary,processes-samples}] [--] command [arguments ...]

Gathers metrics on a command and all its child processes.