phase-list: expose zipmerge via a public API

Expose the implementation of phase zip-merge algorithm via a public API.

A part of https://github.com/strongloop/loopback/issues/895

/to @raymondfeng please review
/cc @ritch 
As you pointed out in https://github.com/strongloop/loopback-phase/pull/8/files#r21615483, some of the orders are not explicit (I cannot derive them until I see your listing). We could simply allow the phases to be spelled out in multiple arrays, such as:
```js
phases: [
  ['initial, 'session', 'auth'],
  ['initial', 'postinit', 'preauth', 'auth'],
  ['postinit', 'session']
  ['preauth', 'session']
];
```