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
  tmux/tmux@9fe43d6
• copy-mode and copy-mode-vi as described in tmux/tmux@76d6d36

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: #13

PR welcome to address other changes.