Check if a file has changed before overwriting

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As discussed in the readme, currently pytest-accept will overwrite a file unless --accept-copy is passed.

Generally that's fine. But recently I've been using it as a realtime tool recently (i.e. it runs with pytest -f --accept, running on every save), which is really cool. And it requires pausing on each save for it to generate and write new results, and getting that wrong causes an overwrite and resolving the change in an editor (or losing it), which can get confusing, even if it's only a couple of seconds of changes.

I think by default it should check that the file hasn't changed before overwriting. If it's changed, it still fails the test as it would otherwise, and prints a message saying it didn't overwrite and suggesting to run again

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Closed by #17