Add support for Fashion MNIST dataset

[quietlychris]

@davidMcneil

Hello,

This pull request adds initial support for the Fashion MNIST dataset, which is similarly-formatted. It includes:

• Extensions to the download features for downloading the FMNIST tarballs,
• Updates the library to the Rust 2018 edition
• Version updates of dependencies (notably, the reqwest crate has changed since async/await has shipped, although the get() function is still blocking)
• Adds an example that displays parsed images using the minifb library.

All tests should still pass, and installation instructions for the dependency on the optional example are included in the README

[quietlychris]

Also, I found an issue with creating the download directory with arbitrary base directories, which appears to have been solved by switching out create_dir() with create_dir_all(), per commit 5f734af

[davidMcneil]

Awesome! Thanks for the contribution!

[quietlychris]

Of course :) I hope some people will find it useful!