Actually use `a:option` instead of `'set-clipboard'` string when
comparing the tmux option value. This hasn't been an issue because the
only use of the function happens to be checking the value of
`'set-clipboard'`.
Move logic to `autoload/tmux.vim` to detect the `vim` is being run
inside a `tmux` sessions and if the `set-clipboard` option is enabled.
This cleans up adding the optional `oscyank.vim` plugin when needed.
Make using the `@vim$TMUX_PANE` window flag much more robust by
unsetting it every time the zsh prompt is drawn and setting it again
using a `FocusGained` `autocmd` in vim. This removes the need for the
hacky check for zsh in the conditional defined in the `in_vim` variable.
