clarifies stream seeking for querying and detection functions

[ssfrr]

The query infrastructure was already handling seeking around the stream
before and after calling the detect functions, so they shouldn't have to
worry about it. This also sets the example in the README to skip the magic
bytes in load(::Stream) instead of load(::File).

fixes #163

[timholy]

Ugh, I just noticed this. If you want something like this it will likely be easier to start from scratch. I am not 100% certain we've been as careful as we should be about the stream position upon exit, or even what the right default really is. Maybe the detector should leave the position as-is after successful detection?

Happy to help you get this over the finish line if you're still interested.