Customise how the store labels appear in dropdown

Problem/Motivation

Be able to customize whether to show the store label or currency in the dropdown.

Proposed resolution

Add configuration options on the block to be able to customize whether to show just the label/currency or both.

User interface changes

Two checkboxes in a fieldset in the block will appear to be checked as to whether to show the label or currency (or can check both to show both).

This may be quite an opinionated patch and it may not fit most people's use case, and I was just going to preprocess the options and override the currencies, but for performance reasons I wanted to do it at this level. We wanted to just show the currencies in our drop down, so I've added the functionality to be able to do so, or show both, or just the store label!

This may not be useful to everyone but just popping it here. :-)