code: function Unshort(options) {
    if (!(this instanceof Unshort)) return new Unshort(options);
  
    options = options || {};
  
    this._options = merge.all(
      [
        {
          timeout: 30 * 1000,
          retries: 1,
          followRedirect: false,  // redirects are handled manually
          maxBuffer: 1000 * 1024, // too big reply -> not redirect
          headers: {
            'User-Agent': defaultAgent
          }
        },
        options.request || {}
      ],
      { clone: true }
    );
  
    // config data with compiled regexps and fetch functions attached
    this._domains = [];
  
    this.cache = options.cache || {
      get() { return Promise.resolve(); },
      set() { return Promise.resolve(); }
    };
  
    this.nesting = options.nesting || 3;
  
    // Regexp that matches links to all the known services, it is used
    // to determine whether url should be processed at all or not.
    //
    // Initialized to regexp that matches nothing, it gets overwritten
    // when domains are added.
    //
    this._matchAllRE = /(?!)/;
  
    let customProviders = requireAll(path.join(__dirname, 'providers'));
  
    let domainMap = {};
  
    config.forEach(domain => {
      if (typeof domain === 'string') {
        domainMap[domain] = {};
        return;
      }
  
      Object.keys(domain).forEach(d => {
        domainMap[d] = Object.assign({}, domain[d]);
      });
    });
  
    Object.keys(domainMap).forEach(domain => {
      if (customProviders[domain]) {
        domainMap[domain].fetch = customProviders[domain].fetch;
        domainMap[domain].validate = customProviders[domain].validate;
      }
  
      this.add(domain, domainMap[domain]);
    });
  }
docs: Create an unshortener instance  options: - cache   (Object) - cache instance - get(key) -> Promise - set(key, value)  -> Promise - nesting (Number) - max amount of redirects to follow, default: `3` - request (Object) - default options for `got` in `.request()` method