Absl flags pathlib support

Are there any plans to support a `flags.DEFINE_path` or something similar that utilizes python's `pathlib` module to support declaring `os.PathLike` operations for the default option?

For context, I often find myself doing the following:

```py
from absl import app, flags
from pathlib import Path

FLAGS = flags.FLAGS
flags.DEFINE_string("filepath", "path/to/some.txt", "Input filepath for program")

def main(argv_):
# Override FLAGS.filepath with a pathlib.Path version
FLAGS.filepath = Path(FLAGS.filepath)

# Rest of program uses pathlib for path operations

if __name__ == "__main__":
app.run(main)
```

It would be awesome if there was an easy way to define a `Path` object from the `DEFINE_` definition in the first place. If adding such a feature is not feasible, I'm curious how other people are using `pathlib` with `absl.flags` as well.

Thanks!

Look at the absl/flags/_defines.py module to see how flag types like DEFINE_string are implemented. Understand the validator and parser pattern. The new flag type should accept a string and convert it to a pathlib.Path object. Check existing tests in absl/flags/tests/ to see how to add a test for the new DEFINE_path flag.

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