Nested keys in $contains operator

[clue]

Now that the $contains operator has been implemented (#9), we should consider if it should support nested keys (dot-notation) like this:

{
    "location" : {
        "$contains" : "home.latitude"
    }
}

This can commonly be expressed like this (which is already supported):

{
    "location.home" : {
        "$contains" : "latitude"
    }
}

This might be particularly useful with regards to #13:

{
    "$contains" : "location.home.latitude"
}

[clue]

This is now explained in the current spec. Postponing this and removing milestone for now.