Add --no-posts flag (or similar)

[thisisparker]

Sometimes you just wanna update the database and don't want to post. (I'm thinking specifically of just after adding a new RSS feed, but there might be other applications.) There should be an option to suppress public posts (or, alternately, a way of triggering such a dry-run when adding a new feed... could be part of the eventual feed-config-in-app flow).