feature: hankaku-kana (half-width kana) support

[exoego]

It would be handy if this crate supports hankaku-kana (half-width) to hiragana, zenkaku-kana (full-width kana) and romaji.

"ｱｲｳｴｵ".to_kana(); // ｱｲｳｴｵ, as it is
"ｱｲｳｴｵ".to_hiragana(); // あいうえお. There is no half-width hiragana
"ｱｲｳｴｵ".to_katakana(); // アイウエオ. Zenkaku-kana, or full-width kana
"ｱｲｳｴｵ".to_romaji(); // AIUEO 

I will send a pull request to implement this if this feature is reasonable.

Reject this if it is out of scope 🙇

[PSeitz]

Yes, that seems reasonable!