`log` usages complicates using as a library

Hi! In the vein of #175 I was playing with `fscrypt` as a library to get information about a directory. One thing I noticed, is a bunch of debugging oriented logging. To disable it, I ended up wrapping my calls to fscrypt with:
```golang
origLog := log.Writer()
defer func() {
log.SetOutput(origLog)
}()
```

But that feels a bit messy? I'm not sure if y'all have any thoughts about this, but I wanted to mention it as a small annoyance.