scrub plugin: preserve tags

I just happened to run into the following issue: One of my plugins taps into the `write` event to [preserve the file's year tag](https://github.com/sampsyo/beets/issues/1498). In the event handler, I read the file's tags again, constructing a `MediaFile` object. While `import`ing new songs, I noticed that the properties of those MediaFiles were returning `None` (e.g. `mf.year == None`), although they should contain valid values. Turns out that disabling the `scrub` plugin fixes this.
Now what I think is happening here is that the `scrub` plugin comes into action and deletes the file's meta data right before my other plugin kicks in, only to find empty tags.

IMO there are two ways to fix this:
- Make the execution order of plugins configurable (probably a smart idea anyways, but could be a big chunk of work)
- Use a white list in `scrub`, permitting to preserve listed tags. Something along the lines of:

```
scrub:
auto: yes
preserve: art comments year
```

I am not sure about the former, but the latter seems not too difficult to achieve. Similar to what we do with the cover art on a manual `scrub` operation: Save listed tags → clear tag frame → restore preserved tags. This would be a more generalized approach to solving #571.

Start by reading the scrub plugin implementation and the handling of the write event, then compare it with the existing manual scrub behavior that preserves cover art. Determine how a configured list such as `preserve: art comments year` should be applied during automatic scrubbing, and verify that plugins reading MediaFile tags during import still see the preserved values.

Written by the indexing model from the issue text.