Add write option to bpm plugin

[jackwilsdon]

Add write option to bpm plugin and tidy up command function.

It looks like a write option was going to be implemented but never ended up being added? The reason I think this is that there's an unused write argument for get_bpm that writes the item after the BPM is updated.

See #1991 and #1991 (comment) for more information.

[sampsyo]

Good idea; thanks!

We actually have some generic machinery for this kind of thing in the form of the beets.ui.should_write() function. The idea is that the import.write option serves as a default, but plugins can override with their own one-off write option too. Here's an example in the mbsync function:

beets/beetsplug/mbsync.py

Line 68 in db36d68

write = ui.should_write(opts.write)

Can we use that so users like @jfunction get what they expect the first time around?

[jackwilsdon]

So should I remove the bpm.write configuration value? Or should I pass it's value to should_write so it can be set either in bpm.write or import.write?

[sampsyo]

Probably the most straightforward way would be to remove that option, yes. If you like, we could also add a command-line override flag like mbsync has.

[Kraymer]

lgtm

[sampsyo]

Hey! @Kraymer's comment reminded me that this was still open. Sorry for letting this slip through the cracks! It looks like this is 100% ready to go (modulo moving the changelog entry)?

[sampsyo]

Merged with changelog entry moved! ✨ Many belated thanks, @jackwilsdon.