add option to disable bom sniffing

[LesnyRumcajs]

#1207

[BurntSushi]

@LesnyRumcajs Thanks so much for this! I ended up re-working this PR a bit and re-submitted it: #1237. I think I might have no explained the right path to take very well. In particular, a problem here is that EncodingMode was added to the public API of the grep-searcher crate and a breaking change was introduced by changing the method signature of the SearcherBuilder::encoding builder. What I had intended was for EncodingMode to be an internal detail used in ripgrep's core src/args.rs. That's what I meant by "forwarding" the bom_sniffing option that you added to encoding_rs_io up through grep-searcher::SearcherBuilder.

More subtle than that was that when BOM sniffing was disabled, the BOM was still being stripped, and I don't think that's correct since the point of this is to get at the raw bytes directly, even if there's a BOM and including the BOM. This led to uncovering another subtle but in encoding_rs_io, which I fixed in this commit.

Thanks again for your work on this. Mostly everything you wrote I kept; it just needed to be shuffled around a bit. :-)