TypedDict is not supported

Hi there,

I'm new to datafiles but found it very useful.
However, my codebase has a TypedDict, which causes an error.

```
from datafiles import datafile
from dataclasses import dataclass
from typing import TypedDict # for Python 3.8 or newer
# from typing_extensions import TypedDict # for Python 3.7 or older

class Job(TypedDict):
title: str
salary: int

@datafile("{self.name}.yml")
class Person:
name: str
nationality: str
job: Job

Person("Bob", "UK", {"name":"CEO", "salary": 999999999})
```

I got the following error:
```
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
[](https://localhost:8080/#) in ()
15 job: Job
16
---> 17 Person("Bob", "UK", {"name":"CEO", "salary": 999999999})

3 frames
[/usr/local/lib/python3.7/dist-packages/datafiles/model.py](https://localhost:8080/#) in modified_init(self, *args, **kwargs)
81 with hooks.disabled():
82 init(self, *args, **kwargs)
---> 83 Model.__post_init__(self)
84
85 cls.__init__ = modified_init

[/usr/local/lib/python3.7/dist-packages/datafiles/model.py](https://localhost:8080/#) in __post_init__(self)
16 log.debug(f"Initializing {self.__class__} object")
17
---> 18 self.datafile = create_mapper(self)
19
20 if settings.HOOKS_ENABLED:

[/usr/local/lib/python3.7/dist-packages/datafiles/mapper.py](https://localhost:8080/#) in create_mapper(obj, root)
286 self_name = f"self.{field.name}"
287 if pattern is None or self_name not in pattern:
--> 288 attrs[field.name] = map_type(resolve(field.type, obj), name=field.name) # type: ignore
289
290 return Mapper(

[/usr/local/lib/python3.7/dist-packages/datafiles/converters/__init__.py](https://localhost:8080/#) in map_type(cls, name, item_cls)
165 return Enumeration.of_type(cls)
166
--> 167 raise TypeError(f"Could not map type: {cls}")

TypeError: Could not map type:
```

Start in datafiles/mapper.py at the create_mapper call shown in the traceback, then follow map_type in datafiles/converters/__init__.py where the TypedDict error is raised. Run the provided TypedDict example while tracing that path; done means the example no longer raises the mapping TypeError and the TypedDict fields are handled.

Written by the indexing model from the issue text.