Implement subcommands

Implement git-style subcommands:

``` r
sys$cmdline$parse(cmd('install', install,
arg('name', 'the module to install'),
opt('f', 'force', 'force override the target')),
cmd('uninstall', uninstall,
arg('name', 'the module to uninstall')))
```

With input `'install target --force'` would call `install(name = 'target', force = TRUE)`.

Start by tracing sys$cmdline$parse and the cmd, arg, and opt entry points shown in the issue. Implement dispatch based on the first subcommand, positional name handling, and the force option; done means the install example calls install(name = 'target', force = TRUE) and uninstall accepts its name argument.

Written by the indexing model from the issue text.