Allow alternative placeholders for unknown characters

[epage]

From the docs

/// * Some Unicode characters transliterate to an empty string, either on purpose
/// or because deunicode does not know about the character.
/// * Some Unicode characters are unknown and transliterate to "[?]".

1. It isn't clear when an empty string vs [?] is used
2. I might want to do something different here, either detect and report an error to the user, put in my own character, etc.

A limited workaround exists

let custom_replacement = "?";
"string".chars().map(|c|deunicode_char(c).unwrap_or(custom_replacement)).collect::<String>()

but deunicode does a lot more than that (spacing logic).

(fork of chowdhurya#4)

[epage]

Originally I wanted to use my own character but I am using this for URLs and need to maintain compatibility, so I think erroring to the user and requiring them to give me a slug would be better.

[kornelski]

ok, done in 0a850a8