Added category selection to the trivia game

[ImMortaLGuruji]

Added the feature where you can select a category in trivia to get a question from.

Changes

• A new option named "category" added to the options for the trivia game. Using this option users can select a preferred category to get the question from. If this option is left null then a random category is selected.
• I have also added error checks for this option so the new code does not contain any bugs.
• Also updated examples to have the new category option.