deepProperties support for relative and absolute pointers

Hello,

From the readme for deepProperties:
This keyword allows to validate deep properties (identified by JSON pointers).

I have a need to validate 'cousin' properties inside an array. I've tried this:

```
let schema = {
deepProperties : { "/parent/child1/grandchild1" : {
deepProperties : { "2/child2/grandchild2" : { const : true }}
}}
};

let data = {
parent : {
child1 : {
grandchild1: true
},
child2 : {
grandchild2: false
}
}
};
```

And got: keyword schema is invalid: data should match format "json-pointer", data property name '2/child2/grandchild2' is invalid.

Tracing a bit - it looks like the validation code from ajv/lib doesn't support relative pointers.
I can build another custom keyword to wrap deepProperties I think, but wanted to report this either for fixing or for clarifying.

Thanks!

Koby