Add support for data-only notifications

[zbarbuto]

Description

Adds a 'dataOnly' key which allows creating android notifications using only data properties instead of notification properties.

The specific use-case here is allowing "data-only" notification payloads as "notification" do not forward the data:

https://github.com/phonegap/phonegap-plugin-push/blob/master/docs/PAYLOAD.md#notification-vs-data-payloads

Related issues

• closes #165

Checklist

• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide

Note, I avoided the ES6 syntax as the lint file still specifies ES5 only

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[zbarbuto]

Not stale - still waiting for review...

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[zbarbuto]

Not stale - still waiting for review...

[dhmlau]

@strongloop/lb-next-dev , PTAL.

[bajtos]

@zbarbuto thank you for the pull request and sorry for keeping you waiting for so long. We are not closely watching this repository these day, since most of our effort goes to LoopBack 4.