Port pipelines-limit flag from dspash-future branch

[angelhof]

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.