add ability to set custom css classes on block

[danmarsden]

Moodle's html/text block allows us to set custom classes to the block - we should reproduce that functionality within the multiblock plugin.

When creating a user tour in Moodle - the admin can select a block "type" eg "multiblock" but it highlights all blocks of that type on the page. If you want to select a single block it's quite hard to target using a simple css rule - particularly if you are using a dashboard style page as the instanceid of the block will be different on every users dashboard.

If we can add a custom class to the block it would make it easier to target in something like a user tour, but also easier to style a specific block within the site - even when used on a dashboard page.