Improve handling of command descriptions/help

[gjwgit]

This needs some requirements specification in the first instance

The YAML file needs to capture the command line arguments for each command, and allow a brief description.

This information is then to be displayed with, e.g.,

$ ml geocode bing --help

  $ ml geocode bing [options] <address>

    Generate latitude and longitude and other information 
    for the supplied address. 

    Options include 
        --max=5    maximum number of matches returned
        --url,--osm generate open street map url

See https://mlhub.ai/survivor/commands.html

[gjwgit]

See also #267