feat: Allow max concurrency for parallel map with option

[weswigham]

Makes map aware of a NAL_MAX_CONCURRENCY environment variable which, when set to a positive integer, controls the maximum number of concurrent iterations which may be in-flight at once.

My usecase here is that I'd like to see a deterministic build log output order from a build I don't really control, which is run with either gulp or just (both of which use undertaker, which uses bach which uses this for its parallel mode). To do so, I'd like to set the maximum allowable concurrency to 1 concurrent operation at a time, this way the output log order isn't affected by the runtime of various operations. lerna and rush both allow doing this at the monorepo level, however there's no way to do it down at the individual task-runner level yet.

If this would be a welcome change to now-and-later, I can add some tests and documentation, or, if you think a "no concurrency" type feature would be better served in undertaker itself, I could look there, too (although at any layer above this you pretty much just get a yes/no, not a configurable maximum count, unless you amend now-and-later's API to take an optional concurrency limit parameter).

[weswigham]

Huh. I am unsure what I am doing that would only fail on node 0.10... Is there some old v8 bug I'm triggering?

[phated]

@weswigham I think it's useful to support max concurrency in here but I don't want to use an environment variable. Can you change the PR to pass in maxConcurrency as an option?

[weswigham]

@phated should the option be a member of the extensions object?

[phated]

@weswigham yes, I think we can include it in the extensions object and then I will rename them in the docs.

[phated]

This PR seemed to have stalled, so I updated it with my desired API, changed extensions to options throughout, added tests and docs.

I also rebased this on the new project template updates.

If everything is green, I'll get this merged 🎉 Thanks for sending over the proposal @weswigham