Make populate param multilevel

[jaysalvat]

Hello.
The populate params didn't behave as expected. Multiple levels should be handled in order to get relations in relations populated.

Here is a PR to help :)

?populate=level.level1.level2&fields=level.level1.name,level.level1.level2.age

population: {
    path: 'level',
    populate: {
        path: 'level1',
        select: 'name'
        populate: {
            path: 'level2',
            select: 'age'
        },
    },
}

[coveralls]

Coverage remained the same at 100.0% when pulling 5f28d3f on jaysalvat:master into a744807 on loris:master.

[jaysalvat]

Hello!
Do you plan to merge this PR?
Can I do something to make it better?
Thanks.

[jaysalvat]

Ping :)

[loris]

Hi @jaysalvat Sorry for the delay, for whatever reason I had github email notifications off on that repo. Thanks for the PR! Publishing to npm right now.