Add className option for toolbar items

[mkaz]

Allows for passing in a className when registering a toolbar button to allow to target via CSS.

An example:

   Prism.plugins.toolbar.registerButton( 'my-button', {
        text: 'My Button',
        className: 'my-button',
    });

This will add the span element <span class="my-button">My Button</span>

[RunDevelopment]

@mkaz Thank you for this PR!

Please also adjust the type definition of opts and the formatting (no leading/trailing spaces inside round braces).

[mkaz]

@RunDevelopment Updated my PR per review, thanks!

[mAAdhaTTah]

One minor nit but lgtm!

[mAAdhaTTah]

Thank you for the contribution!