Support replacement string rather than erroring for invalid input?

[nigels-com]

Some discussion over at fluent-bit about supporting more modes than erroring for invalid input.

Could we perhaps add a const char * replacement parameter, which would be an UTF-8 string to indicated invalid input characters. If NULL, then error. If empty-string, then skip and continue.

Would also need to factor into the length logic.

[nigels-com]

Done.