Adding @yank_selection option to override xclip selection

I needed to change from the clipboard selection to primary to get tmux-yank working well with my xmonad environment.  This appears to retain your default and let the user change via the ```@yank_clipboard``` option in their ```~/.tmux.conf```.

Here is an example of my config:

```
set -g @yank_selection primary

# List of plugins
# Supports `github_username/repo` or full git URLs
set -g @tpm_plugins "              \
  tmux-plugins/tpm                 \
  tmux-plugins/tmux-yank           \
"
```