Custom CSS class per trial

From #831:

This is a design issue with the library that we’ve been thinking about for some time. I think we’re likely to add a feature in the future that adds a custom class to the display element so that styles can be specified in a <style> tag or stylesheet.

The feature might look like this:

var trial = {
	type: 'html-keyboard-response',
	stimulus: 'apple',
	css_classes: ['large-text']
}

And that would add the class .large-text to the root HTML element of jsPsych for that trial. Then one could specify CSS rules that applied to that class.

[jodeleeuw]

🚀 merge away!

[becky-gilbert]

I added a first draft of a new “Style and Formatting” page to go in the Overview section of the docs: https://github.com/jspsych/jsPsych/blob/feature-css-class-trial/docs/overview/style.md I may have gone overboard - sorry 😬 Please feel free to comment/edit mercilessly!

I also made some changes to the “css-classes-parameter.html” example file, such as adding examples that show how to write more specific CSS selectors, as explained in your earlier comment. Again, happy to get feedback on how to improve this example file.