title: How can we override the getInputType function in angular2-query-builder
body: <p>I'm using angular material code of angular2-query-builder in my project and I'm facing an issue where I have to dynamically change the config.field based on the selection from a previous screen.
  When I change the fields, I'm able to see the new fields but, the browser console throws the following error :</p>
  <pre><code>    QueryBuilderComponent.html:109 ERROR Error: No configuration for field 'Customer Name' could be found! Please add it to config.fields.
      at QueryBuilderComponent.push../node_modules/angular2-query-builder/dist/components/query-builder/query-builder.component.js.QueryBuilderComponent.getInputType (query-builder.component.js:228)
      at QueryBuilderComponent.push../node_modules/angular2-query-builder/dist/components/query-builder/query-builder.component.js.QueryBuilderComponent.findTemplateForRule (query-builder.component.js:165)
      at Object.eval [as updateDirectives] (QueryBuilderComponent.html:124)
      at Object.debugUpdateDirectives [as updateDirectives] (core.js:11054)
      at checkAndUpdateView (core.js:10451)
      at callViewAction (core.js:10692)
      at execEmbeddedViewsAction (core.js:10655)
      at checkAndUpdateView (core.js:10452)
      at callViewAction (core.js:10692)
      at execEmbeddedViewsAction (core.js:10655) 
  </code></pre>
  <p>Upon debugging, it is coming from QueryBuilderComponent.getInputType
  I want to bypass this condition and return null if field is not found:</p>
  <pre><code>QueryBuilderComponent.prototype.getInputType = function (field, operator) {
          if (this.config.getInputType) {
              return this.config.getInputType(field, operator);
          }
          if (!this.config.fields[field]) {
              return null; //MY CODE
              // throw new Error(&quot;No configuration for field '&quot; + field + &quot;' could be found! Please add it to config.fields.&quot;); // EXISTING CODE
          }
          var type = this.config.fields[field].type;
          switch (operator) {
              case 'is null':
              case 'is not null':
                  return null; // No displayed component
              case 'in':
              case 'not in':
                  return type === 'category' || type === 'boolean' ? 'multiselect' : type;
              default:
                  return type;
          }
      };
  </code></pre>
  <p>I can't change this in QueryBuilderComponent.js file in node_modules. How can I override or customize this &quot;getInputType&quot; function.</p>