Trigger User events when table mode is enabled or disabled

This can be used by the user to perform custom actions when table-mode
is enabled or disabled.

This has in mind vim-pandoc's use case, where we would want to toggle
autoformatting depending on whether table-mode is enabled or not (see
https://github.com/vim-pandoc/vim-pandoc/issues/95). With this, we could
simply do

    au User table-mode-enabled call
pandoc#formatting#DisableAutoformat()
    au User table-mode-disabled call
pandoc#formatting#EnableAutoformat()