add window.ZeroMdCssUrls hook for default CSS

[bennypowers]

Problem:

There's no current way to set a default theme. You end up having to copy/paste css-urls="['my-theme.css']" in to every instance of the element

Proposed Solution

Expose an optional ZeroMdCssUrls hook on the window object

Some Possible Objections

Pollutes window, but then again, there's already ZeroMdStore

Alternative Solutions

default to a shipped file, (i.e. 'zero-md/defualt-theme.css') tell users to overwrite that file

[zerodevx]

Thanks for the PR! Agree that some mechanism should be exposed to allow global settings. Let me take a look and get back to you.

[zerodevx]

@bennypowers Thanks again for the PR - I thought it makes sense to expose all configurations (not just css-urls) through the ZeroMd.config hook instead.

This also gives the opportunity to refactor ZeroMdStore into ZeroMd.cache - so that all zero-mdish can be encapsulated inside the ZeroMd global.

Hopefully this fits your requirements!

[bennypowers]

LGTM
Closing in favour of #14