Add support for blocking contacts and accounts (#260)

* Add blockContact and unblockContact subcommands

* Send blocked status in contacts sync

* Use only one method for blocking and unblocking

* Add blocking/unblocking for groups

* Prevent blocked messages from being printed

* Print blocked property in listContacts and listGroups commands

* Handle BlockedListMessages

* Store blocked state from incoming contact and group sync messages

* Minor changes and corrections

* Add block and unblock commands to man file (and also fix some headings of commands)