Use `if` instead of `run` for the tmux bindings

Closes: https://github.com/christoomey/vim-tmux-navigator/pull/31
Update README, add `TmuxPaneShowEnvVar`

Remove `TmuxPaneCurrentCommand`, which is now obsolete.
Vim plugin: add/export `TmuxNavigateInit`

Encapsulate the init procedure into a function, which could be useful
when debugging.
Merge branch 'use-tmux-env'