Handle cross-origin errors when parsing styles

[franga2000]

When using external stylesheets, the site can't access their contents due to cross-origin request restriction, which completely breaks the form.

This PR handles two cases:

1. A top-level stylesheet (<link rel="stylesheet" href="...">) is loaded from a different origin (like a CDN):
   in this case, a warning is printed instructing the developer to add crossorigin="anonymous" to the <link>, which fixes the issue

2. An accessible (same-origin or crossorigin="anyonymous") stylesheet loads an external stylesheet via @import:
   in this case, no workaround exists (that I know of), but a warning is still printed to let the developer know

(Fixes #39)

[jrief]

interesting use case. I generally advise against including CSS and JS from a CDN.

[jrief]

interesting use case. I generally advise against including CSS and JS from a CDN.