events should default to last ledger

[leighmcculloch]

The stellar events command requires a ledger or cursor to be specified.

However the CLI provides no utilities for getting the last ledger, or information about a network. There is this open issue to do that:

• https://github.com/stellar/stellar-cli/issues/1612

But also, it's reasonable that someone should be able to run the events subcommand and retrieve events automatically from some point, probably the most recent ledger.

[janewang]

@leighmcculloch I agree with not requiring a last ledger to be specified.

CLI should also be able to provide information like getLatestLedger or getTransaction