feat: fallback CDN

[mastilver]

inspired by jso0/html-webpack-cdn-plugin#1

Depends on #13

---

options.fallbackResolver

Type: string || function

Default: 'module-to-cdn' || 'module-to-jsdelivr'

Similar to resolver but the module will be loaded from this CDN only if the main CDN is experiencing issues

---

It will create a new file which will contains for each module:

window.React || document.write('<script src="https://unpkg.com/react@15.6.1/dist/react.js"><\/script>');

NOTE: resolver and fallback must both return a valid cdnConfig otherwise the module is ignored

[mastilver]

This would be a better way as it would virtually handles all types of resources, not only JavaScript:

var fallbackUrls = {
  'https://unpkg.com/react@15.6.1/dist/react.js': 'https://cdn.jsdelivr.net/npm/react@15.6.1/dist/react.min.js'
};

window.addEventListener('error', function(e) {
    var failedUrl = e.target.src || e.target.href

    if (failedUrl in fallbackUrls) {
        if (isJs) {
            document.write('<script src="https://unpkg.com/react@15.6.1/dist/react.js"><\/script>');
        } else { ... }
    }
}, true);