Add support for arbitrary escape sequences

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People regularly request support for various escape sequences (e.g., https://github.com/termbox/termbox2/pull/120, https://github.com/termbox/termbox2/issues/119, https://github.com/termbox/termbox2/issues/56). Optimistically, adding support for more sequences makes the library useful to more people. Pessimistically, it bloats the library and makes it less portable (especially for sequences not in terminfo). In practice we take a balanced approach. A third option beyond saying "yes" or "no" to these requests is to add support for extract sequences via a callback. This was the intent of TB_FUNC_EXTRACT_* but I messed up the implementation. (It's now deprecated and will be removed in termbox2 v3.) We should take another shot at that though.