ledger: Dedicated reporting tab for Expenses:Payroll.

To better accommodate the new payroll-type metadata.

ledger: Only display accounts requested with --account.

Now that we're accepting classifications, it's possible to specify account
options that select some but not all accounts at the same level of the
hierarchy. This commit tracks requested account names separately from sheet
names to do that correctly.

ledger: --account accepts a classification.

This makes it easier for users to specify a group of accounts.