Adding promise support using universalify

[kartik2406]

I saw your comment on this pull request, #103.
I used universalify to add promisesupport. Must admit the solution looks very clean this way.
Could you please review this?

Things changed:

1. Added universalify to package.json
2. Imported universalify in index and created promise based functions for read and write File
3. ran npm test, got standar erros
4. ran standard --fix, hence the reformatting

Could you tell me if i need to change something or if the changes look good to you.

[RyanZim]

Please remove the package-lock.json.

[kartik2406]

Done 😄

[kartik2406]

Hey i also added documentation in read-me. Could you please check that out too?

[RyanZim]

Also should add tests for promise usage.

[kartik2406]

@RyanZim sure i will add tests. I had one doubt though.
For few tests, one example "> when invalid JSON and throws set to false"
there have been two calls made to the function, one with and one without options argument. As per my understanding we don't need to check without the options lag in this case right? or am i missing something here?

[kartik2406]

I have added tests for promises, also have refactored existing tests so that common code can be shared among callback and promise tests (using beforeEach)

[RyanZim]

LGTM

[RyanZim]

Released in 5.0.0 🎉

[kartik2406]

yaay 😄 🎉