Add an option to allow certain modules to search upwards

[sindresorhus]

I had the need to allow a specific module to require upwards, so I added a private option _allowedMethods in 64ac0d0. I made it private because I'm not totally happy with it. It only works for top-level imports. So you can allow require('a') to be searched upwards by including a in the option, but if a imports a module name b, there's no way to allow b specifically, other than allowing any module from a.

Context: atomiclabs/hyperdex@d54a47c