Path join feature

I'd like to propose a method to join paths, i.e. add/concatenate a path to an existing one:

```ruby
JsonPath.new("$.store").join("book..author").on(document)
# => JsonPath.new("$.store.book.author")
```

The use case i encounter quite often is: in, e.g. unit tests, i use some kind of base path, and i want to extend it multiple times and for different json documents. As i want to stay DRY i can only concatenate strings. However, a #join method provided by JsonPath would be IMHO much cleaner.

Thanks for your feedback/review.