Force checking accounts and payees.

Add options --strict, --check-payees, and --explicit to force checking accounts and payees. We use --strict instead of --pedantic because we do not want to raise errors, but just warnings. The --explicit option forces checking cleared transactions (Ledger's default is to check only uncleared transactions). See also: http://article.gmane.org/gmane.comp.finance.ledger.general/4737/match=strict+cleared+transactions.