consider making definition of line terminator configurable

[BurntSushi]

In particular, it can sometimes be useful to set it to things like \x00. See BurntSushi/ripgrep#1487 for a use case.

Note that this is orthogonal to #244. It should be easy to be able to set the line terminator to any single byte. Although, the API may want to consider the possibility of #244 being fixed some day as well.

[BurntSushi]

This will be available as an option, at minimum, for the meta regex engine in regex-automata 0.3, which I hope to release soon.

I probably won't make this an option in the regex crate proper, but would be open to it if someone had a use case for it.