Separate CLI from package.json:main

[exogen]

This PR offers protection against misbehaving dependencies as discovered in #17.

The issue is that some dependency, e.g. modify-babel-preset, might attempt to require() everything under node_modules as part of a module resolution strategy. This assumes that require() on any given module has no side effects (like running a script, for example, as postinstall-build does).

So this separates package.json:main (index.js) from package.json:bin, which now points to cli.js. Thus calling require('postinstall-build') will no longer do the same thing as running postinstall-build.

Fixes #17. /cc @b1r3k