Simplify rib CLI commands

[srid]

[aka. Don't use sub-commands when they aren't necessary for the task]

Instead of having multiple sub-commands, each taking their own options, simply have one implicit top-level command, that takes a list of options.

That is,

$ ./result/bin/rib-sample  # Generates the site once
$ ./result/bin/rib-sample --watch # Generates, and regenerates when source dir changes
$ ./result/bin/rib-sample --watch --serve # Also serves the output directory

This idea has been brewing for a while now, but #143 finally nudged me to file an issue for it.