Automatically pull bigram-files from websites

[Glitchy-Tozier]

From @timeopochin, first mentioned in #3:

I also thought I could add an option to pull the files automatically from practicalcryptography. This would take away some of the hassle of getting the files without removing the ability to provide your own.

1. I agree that this would be nice to have.
2. However, I think it might be too narrow of a functionality, when targeting only practicalcryptography
3. Additionally, I'm not sure how legal this would be. This is the same reason why I didn't add their files to the repo.

However, something that might work is some kind of general functionality where a user can request the optimizer to grab the bigrams from any URL. The default URLs could then be the ones from practicalcryptography.