An option for choosing custom npm client

[oMaten]

For now, ntl use npm to execute script by default. I need to use private npm client to execute script for some reasons(like preset npm remote registry, etc...), so is it possible to add an option for setting private npm client?

[ruyadorno]

that's something I had in the back of my mind for a while but never really had the time to work on it

I think it's definitely very nice to have 👍

[Calinou]

I suppose this would also be useful to run scripts using Yarn instead of npm, which can be slightly faster in my experience. I guess this should this be configured using an environment variable (NTL_RUNNER?).

[ruyadorno]

Landed in #49