try to load sub-command with require('mojito' + subcmd)

note that this means that users who install sub-commands globally also need
to set $NODE_PATH to contain `npm root -g`. The natural set of paths node
looks in otherwise doesn't include the place where npm libs go with
`npm i -g` (which is distinct from executables in $PATH).

    % node -pe module.paths
    [ '/Users/isao/Repos/wip/clistuff/node_modules',
      '/Users/isao/Repos/wip/node_modules',
      '/Users/isao/Repos/node_modules',
      '/Users/isao/node_modules',
      '/Users/node_modules',
      '/node_modules' ]
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