Added a select all feature

[Nathan-Roberts123]

What issue does this pull request resolve? This will pull request will add a new feature that was requested by users, this feature allows a user to click on a select all button to select all options in a multiple select input. Issue #821

What changes did you make? I added a button to select all options in a multiple input options menu, and I created the function to handle that click and add all options to selectedOptions array. I also wrote a test for the feature.

Is there anything that requires more attention while reviewing? No, I don't think there is.

[Nathan-Roberts123]

The feature looks something like this.