Resource usage:
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By default, "rak" attempts to use all computer's resources as
much as possible.  You can tune some of the parameters related
to parallelism of execution.

--batch[=N]

Indicate how many files should be processed within a single worker
thread at a time.  Defaults to 64 if not specified.  When specified
as a flag without any value, will assume the value 1.

Examples:
# run a search using batches of 1024 files per worker thread
$ rak --batch=1024 foo

# run a search with each file handled by a separate worker thread
$ rak --batch foo

--degree[=N | code]

Indicate how many worker threads should be use maximally (which
usually effectively means the number of CPU cores to be used).
Defaults to the number of CPU cores minus one (with a minimum of
one, obviously).  When specified as a flag without any value,
will assume the value 1.  When specified with a Callable, will
call that Callable with the total number of CPUs available,
and will use the result as the value to apply.

Examples:
# run a search on only one worker thread
$ rak --degree foo

# run a search on maximally 2 worker threads
$ rak --degree=2 foo

# run a search on half of the CPUs maximally
$ rak --degree='*/2' foo
