Vouch request: add periodic shake interval setting (addresses my issue #2988)

[danvincent]

Much obliged!

I've found this auto-shake useful to keep context down in an unobtrusive and "non-destructive" way (yes I know it removes stuff but you know what I mean!) - my branch is here if anyone facnies give it a go https://github.com/danvincent/oh-my-pi/tree/feat/periodic-shake-setting

[can1357]

vouched!