Port `pipelines-limit` flag from dspash-future branch

The `dspash-future` branch contains a useful option that puts a maximum limit on the numbers of parallel pipelines that can be spawned by PaSh (https://github.com/binpash/pash/blob/dspash-future/compiler/config.py#L142). It is currently not implemented in `future` and it would be great if the option and its effects are ported to `future`. To resolve this issue it would be good to also add an example usecase for this option in `docs/tutorial.md`.

Read the `pipelines-limit` option at `compiler/config.py#L142` on the `dspash-future` branch, then compare the `future` branch's pipeline-spawning path to identify where the option's effects belong. Add an example to `docs/tutorial.md` and verify that the example demonstrates a maximum number of parallel pipelines.

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