update: Option to exclude fields (the inverse of -F)

I want to write a script that executes `beet update` and `beet write` once a day and I just tested it with the pretend option like this:
`beet update -p /path/to/musicroot`
This gave me mainly changes about the tags `bitrate` and `bpm` like so:
```
50 Cent - Get Rich or Die Tryin’ - Gotta Make It to Heaven
bpm: 92.0056304932 -> 92
```
The same was with `beet write` but the other way around:
```
50 Cent - Get Rich or Die Tryin’ - Gotta Make It to Heaven
bpm: 92 -> 92.0056304932
```
Now I don't want these tags to to be written from the files tags to the database and back since this is not necessary.

I know there is a `-F` option for the `beet update` command but it would be way more comfortable if you could specify the tags you **don't** want to change in the database or in the files' tags.

Does it even make sense to run `beet update` and `beet write` after one another?
What do you think about this?

Greets,
/J