Add support for a custom publishing command, fix postpublish script

[AndreyBelym]

Custom publish command might be required, if you need to build a .tgz of a package first, change some attributes (e.g. file permissions), and publish modified .tgz.

\cc @inikulin

[inikulin]

@AndreyBelym I've added you as a collaborator on npm, so you can release new version yourself.