Add optional progress bar for downloads

[bmgxyz]

Previously, when downloading new episodes the script only printed the URL of the current download and provided no ongoing output. These changes cause a basic progress bar to appear as new episodes are downloaded. I used the progress module (see PyPI), which made this pretty simple. Note that the progress bar only appears if the user passes --show-progress or -p.

For example, downloading the latest episode of Back to Work:

(.env) bradley@enterprise ~/c/podfox> podfox download B2W --how-many 1 --show-progress
b2w-484.mp3 |████████████████████████| 51.9 / 51.9 MB  (4.01 MB/s)

I changed the chunk size to 128 kB from 1024 kB so the progress bar output could be a bit more precise. I haven't noticed any speed issues as a result.

Fixes #1