match 'set-option -g @plugin' and 'set -g @plugin' both

[wyntau]

In my tmux.conf, I use set-option -g @plugin, but in this way no plugins will be loaded.

I checked scripts/helpers/plugin_functions.sh, find the script only match 'set -g @plugin', so I make it match 'set -g @plugin' and 'set-option -g @plugin' both.

[knakayama]

:+1:

[bruno-]

Thank you for the contribution and sorry for waiting so long for this to get merged.

[wyntau]

Great :+1: