Feature: recursive descent operator

Being discussed to formally be added to the spec here: https://github.com/jmespath/jmespath.py/issues/110

If I can get this library back up to working order including the macros, the next thing I'd like to see is an optional feature to add a non-standard recursive descent operator while the spec above gets discussed. Gated behind a feature like "nonstandard-recursive-descent", it would enable parsing `..` in the same manner as JSONPath and E4X, which expands the flexibility of jmespath.

Once a standard solution became defined the spec, this feature could be sunset or default to the official solution.