Add --analyze mode

[jberkus]

Add --analyze mode to analyze only certain tables which otherwise might not get analyzed promptly, such as very large append-only tables, or very small tables. Would work on a simple time-limit basis.

[qdw]

Seems like it would be just as easy to do a cron job that runs analyzedb on specific tables, unless there's supposed to be some intelligence here that can know when a table is, .e.g., append-only (perhaps by consulting pg_stat_user_tables?)

Overall, this feature seems easy to supplant with human knowledge about the DB.