Use postci script instead of postinstall script if present

[rhwood]

Currently, npm ci runs the postinstall script if it is present, which works well enough, but I would like a little more control after installing during npm ci by running a postci script instead.

My use case is with CI services in a lerna maintained Git repo, where I would like to run lerna bootstrap in a postinstall script, while running lerna bootstrap --ci in a postci script.

(I found a similar request on npm.community.)