New status command

[da2x]

From issue #1740.

Sample output:

39 output files modified since last deployment 4 hours and 31 minutes ago.
4 days and 22 hours to next scheduled post.
78 posts in total, 1 scheduled, and 0 drafts.

Counting new posts is out. Tracking whether last build succeeded seemed less useful when I started working on it, so I dropped that too. Checking for version should be done in version plugin.

[Kwpolska]

not really, at least not in this iteration.