[Feature] change the refresh rate in-app

[mmahmoudian]

Checklist

• I've looked through the documentation and existing open issues for this feature/feature request.

Describe the feature request

I have searched through open and closed issue, pull requests, discussion, and have tried to find this in the documentation, but none of the keywords I used lead to any thread. But in case I have missed it, I apologize for creating a duplicate.

It would be nice to let the user change the refresh-rate (frequency of fetching information) on the fly and in the config. As for key bindings, I can suggest Ctrl+ for increase the frequency and Ctrl- for reducing it. It would be extra nice to see the current set frequency permanently or temporarily.

I have two reason for wanting to slow down the refresh rate:

1. I usually have btm open on a side monitor but it constantly changing is a little bit too distracting. Would be nice if I can ask it to refresh once every 30 secs or something
2. I know it is negligible, but I would like to allocate resources to programs according to my needs, and of my need is to monitor my computer every 30 seconds, then it should use the resources every 30 secs ;)

Design consideration:
I don't know how long fetching all the info from the computer would take, but if it is something small, in the long run it can add up, so it would be nice if this time is subtracted from the wait time, so that all queries are almost perfectly at the same intervals. This is not necessary, by any means.

[ClementTsang]

Already exists as a config field or a flag: https://clementtsang.github.io/bottom/0.8.0/configuration/config-file/flags/

As for on the fly, that sounds reasonable as a config screen thing whenever I get around to doing that (#159).