Minor improvements to completion annotations

@jdtsmith Have been very busy, so I just got around to upgrading the version of Activities in my local config and trying the new completion annotations myself. It looks great, so thanks for working on it.

Before making a v0.8 release, I'd like to suggest that a few minor improvements be made (by someone, if not by you):

1. Compared to the version of `activities-completing-read` from v0.7.2, I've sometimes noticed a bit of delay before the list of activities is displayed. At the moment, I can't reproduce it consistently, so maybe it's not a problem after all. But, if you haven't already, would you mind profiling this function and seeing if there are any obvious places that could be optimized?
2. Would you mind gating the annotations behind an option? Some users might prefer to disable them for various reasons (e.g. if they were on an older system and running without Elisp compilation, maybe it could make a difference).
3. The annotations are displayed on the far right edge of the window, which, in a wide frame, is very far from the name of the annotation. Some other annotations, e.g. like those provided by Marginalia, seem to center-align the annotations, so they aren't as far from the main text. Would you mind applying that to these annotations as well? I think it would be a big usability improvement. :)

No obligation is implied, of course; if you don't have the time or interest to work on these now, that's fine.

Thanks.