support changes introduced in tmux 2.4

[tkothe]

A couple of commands have been changed in tmux 2.4. They are not supported by the current version of vim-tmux.

They seem to mostly relate to copying. The commands I could identify

• copy-pipe-and-cancel was in this commit https://github.com/tmux/tmux/commit/9fe43d6acbc4468e3490f9a93df25b37c7dd18fa
• copy-mode and copy-mode-vi as described in https://github.com/tmux/tmux/commit/76d6d3641f271be1756e41494960d96714e7ee58

syntax highlighting does not work for these commands

There might be more, so I am not claiming completeness of this issue.

[bruno-]

Thanks for reporting. copy-mode and copy-mode-vi were addressed in this PR: https://github.com/tmux-plugins/vim-tmux/pull/13

PR welcome to address other changes.