Adds option for user to use their own, "custom" word list

[sts10]

Addresses #9.

Also implements a (case-sensitive) check for duplicates, plus a check for non-uniform Unicode Normalization, on the user-provided lists, hopefully removing some footguns.

I got generate_passphrase to take both &' static [&str] when using a built-in list and Vec<String> when handling a user-provided list, thanks to some Ugly Rust trickery:

pub fn generate_passphrase<T: AsRef<str> + std::fmt::Display>(
    number_of_words_to_put_in_passphrase: usize,
    separator: &str,
    title_case: bool,
    list: &[T],
) -> String {
    // ...
}

What I'd like to improve in this PR

The bummer with this PR as it stands is how ugly src/main.rs is now.

    // I'd like to have this be one variable (one `let`), but I can't figure out what to type to
    // make it?
    let custom_list = opt
        .custom_list_file_path
        .map(|custom_list_file_path| read_in_custom_list(&custom_list_file_path));
    let built_in_list = fetch_list(opt.list_choice);

    // Since I can't have just one list variable yet,
    // I need to do this to get the legnth of the list
    let list_length = match custom_list {
        Some(ref custom_list) => custom_list.len(),
        None => built_in_list.len(),
    };

and later

 // Generate and print passphrase
        // Again, we have more code than we should because of this pesky list type situation...
        let passphrase = match custom_list {
            Some(ref custom_list) => generate_passphrase(
                number_of_words_to_put_in_passphrase,
                &opt.separator,
                opt.title_case,
                custom_list,
            ),
            None => generate_passphrase(
                number_of_words_to_put_in_passphrase,
                &opt.separator,
                opt.title_case,
                built_in_list,
            ),
        };
        println!("{}", passphrase);

I'd love to have one type List that can handle both &' static [&str] when using a built-in list and Vec<String> when handling a user-provided list.

I'm guessing I'll need to implement a trait of some kind? I'm trying to sketch out the problem in a Rust playground, but it's hard going for me!

[sts10]

Should we merge this, adding code complexity but also the ability for users to use their own word list?

Pros (of merging)

• Allows non-English speakers a way to use Phraze
• Makes Phraze more robust and impressive, thus perhaps attracting more users and more contributors, etc.?

Cons

• Adds to code complexity
• Adds new dependencies

Note: Pgen doesn't accept custom word lists, but passphraseme does.