The RNG blocks follow a slightly different format than the word blocks.
They are wrapped in square brackets instead of curly braces. The format
is as follows:
[cmd:arguments]
`cmd` can be one of either "rng" or "hide". "rng" takes a comma
separated list as its arguments and will choose one option from the
list. Only raw words are accepted here (word blocks are not parsed).
"hide" will randomly hide its arguments in the output string. The
arguments for this command *are* parsed, so {word} and {{new_word}}
blocks can be used.