A couple mostly small patches

Hi, and thank you for mu4e!  I'm just getting started with it and really love it so far.

I found a couple small things that you may want to fold in to the main repo.  The propertization ones are admittedly personal taste, but seemed more consistent with how things looked in the rest of the ui.  And I found I wanted to omit some of the flags in the headers view.  The patch keeps the current behavior but allows customization.

I'm happy to make any changes.  Just let me know.

And thanks again for mu4e!