Support JSON Pointer syntax

[jbohanon]

JSON Pointer syntax support was removed in v3.0. I haven't found any issues or PRs referencing why this option was removed. Was it to simplify the code during the AST rewrite? Our use-case for Inja is in application networking (specifically envoy), where it is quite likely that . will appear in JSON keys such as in this simple example:

{
  "domains": {
    "prod.example.com": "up",
    "dev.example.com": "down"
  }
}

where prior to v3.0, we could have used something like {% if domains/prod.example.com == "up" %}prod is up!!{% endif %}. This is a simplified, contrived example, and while workarounds may be possible on a case-by-case basis, this is not practical for user-supplied templates.

This has blocked us upgrading the dependency thus far, but we are now looking to support more recent features (e.g. set). Can this support be added back?