Allow multiple values for the tmux notifier color_location option

This allows you to set the color of multiple tmux options with the tmux notifier.

For example, I have this in my `~/.guard.rb`:

```ruby
notification(:tmux, {
  :display_message => false,
  :success => 'colour150',
  :failure => 'colour174',
  :pending => 'colour179',
  :color_location => %w[status-left-bg pane-active-border-fg pane-border-fg],
}) if ENV['TMUX']
```


I'm happy to update https://github.com/guard/guard/wiki/System-notifications if this gets merged.