Add pr create --body-file flag

[castaneai]

The --body-file flag helps to create long bodies or multi-line bodies.
In particular, it avoids the "filename or extension is too long" problem in Windows environments.

I refer to the following comments.
#595 (comment)

[mislav]

Thanks, this is great!

I've also added issue create -F for parity and converted the tests to a more focused style of tests that exercises only flag parsing in isolation.

[StanleyGoldman]

@castaneai @mislav This is awesome! I would ❤️ if you ported it to the gh issue edit command.