Concatenate two searches, p.e. first all with "tag:flagged", then "tag:inbox NOT tag:flagged"

[MaxGyver83]

Is your feature request related to a problem? Please describe.
Sometimes I flag an e-mail as a reminder to myself that I have to do something about it. But often I forget these e-mails as soon as they leave the first page when more e-mails arrive. Of course, I can filter for tag:flagged but then I need a reminder that reminds me to filter for my reminders. So it would be nice to have flagged e-mails always on top.

Describe the solution you'd like
An easy and generic solution would be an option to concatenate two (or maybe even more) searches. For example :search tag:flagged, tag:inbox (I don't know if a comma is a appropriate delimiter here). That way alot could do two (or more) notmuch searches and just concatenate the resulting lists.

Describe alternatives you've considered
An alternative, less generic solution could be an option like show_flagged_emails_always_on_top = true.

[pazz]

Very nice idea!
Don't wait for me to implement it any time soon but I'd definitely push something like that if someone sends a PR.