Copy file byte range in native code

We have a need to copy byte ranges of files into new files. We've been able to implement this on top of `read` and `write`, but as you would expect, it's quite slow, probably mostly because of having to cross the bridge so many times (though the base64 encoding is an additional bit of work that wouldn't be necessary at the native level).

I've already implemented a working Java version of `copyFileByteRange`, which takes a position and a length and copies only that segment of a file to a destination path. I'm about to do an Objective-C version as well.

Is this something you'd be interested in accepting as a PR? We don't want to maintain our own version of RNFS, but if you'd accept a PR then I'd be willing to port the changes from our own native module into RNFS.