Add custom filter group_by

[arthurmmedeiros]

Hello there,

I previously added this question, but I realised the code I wrote was just wrong. I fixed it and decided to create a pull request with this new custom filter.

I think this is a useful feature, so I felt I could share and give a small contribution to this library.

I added some tests and I'm happy to add more if necessary.

Thank you.

[hishamco]

What's the problem that you going to fix?

[arthurmmedeiros]

@hishamco sorry, I should have given more details about this pull request in my first comment.

The problem I am trying to solve is to group a list of objects by a certain key. In the tests I used an example where I had a list of users, and I wanted to group these users by type (admin and client).

I hope this makes more sense.

[hishamco]

@sebastienros do you have any objection to introduce this filter, before I review