Add $joinEager query param to fetch relations with JoinEagerAlgorithm

[cdelaorden]

Hi, this makes a quick fix for #9

You can now use $joinEager param in the query object, and this relation will be eager loaded with SQL join, so you can actually filter based on related fields.

NOTE: At the moment it doesn't work too well with $select, because $select refers to fields in the Model table and joinEager will add all the related entities field so ambiguous name can occur.

This can however fixed easily by using the modify parameter in the relation, that allows to include specific fields for the related entity.

Pull request includes code, test, and added --fix to your eslint config so spaces and such are automatically fixed. For tests, I added a new Employee model that belongs to existing Company. That DB setup for tests could probably be extracted into a helper file, but I didn't want to move too many parts around. :)

What do you think? :)

[NotAmaan]

@mcchrish Would you mind reviewing this? Its currently not possible to use eager otherwise. Should also fix #8

[mcchrish]

@cdelaorden @NotAmaan thanks for taking a look at this.

It's been awhile since I've used this lib for a project but feel free to extend and send PRs when you see an improvement can be done.

I can see as well that feathers have moved on to v3. Looking forward for this moved along side it.