Support Tốc ký shortcuts

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Some IMEs, particularly WinVNKey, implement a series of typing shortcuts called Tốc ký chữ Việt, proposed by Trần Tư Bình, that aim to reduce the number of digraphs, trigraphs, and doubled letters that must be entered by hand. For example, Z becomes “Đ” in a single keystroke, UZ can become “uông” in some contexts, and “u” is automatically inserted after “q”.

Hiếu and I were never particularly enamored with this system; it has the same power-user feel as vim key bindings. But if there’s demand along the lines of #60, I guess we could bolt it on as an option, emphatically off by default.