Allow for subclassed types

A little background first, in Ansible there is a `json_query` filter plugin which uses `jmespath` to query json. There is an issue open where users of that plugin get the following error:
```
JMESPathTypeError: In function contains(), invalid type for value: irb.1111, expected one of: ['array', 'string'], received: "unknown"
```
The comment that summarizes the issue is here https://github.com/ansible/ansible/issues/27299#issuecomment-366781707.

The error is caused by the fact that Ansible uses its own types for strings. One of the hacks we've come up with was:
```
jmespath.functions.REVERSE_TYPES_MAP['string'] = jmespath.functions.REVERSE_TYPES_MAP['string'] + ('AnsibleUnicode', 'AnsibleUnsafeText',)
```
which seems weird since I am pretty sure `REVERSE_TYPES_MAP` is not supposed to be public.

is there a proper way to do this? If not, is this something that would be considered to be added?

Thanks!