Allow alternative placeholders for unknown characters

[epage]

From the docs

Some Unicode characters are unknown and transliterate to "[?]".

In my use case, I already have a placeholder character. I'd rather not do a search/replace in strings for [?] in case a user has that. It'd be nice if I was given the option to set what the placeholder would be.

[kornelski]

In my fork I've made deunicode_char() return None instead of "[?]".

[epage]

I think both would be useful.

[kornelski]

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

[epage]

I moved this over to kornelski#1