use(plugin) {
	    if (typeof plugin !== 'function') {
	      throw new FormidableError(
	        '.use: expect `plugin` to be a function',
	        errors.pluginFunction,
	      );
	    }
	    this._plugins.push(plugin.bind(this));
	    return this;
	  }
Write the field `name` and `val`, or multiple fields with one object
for "multipart/form-data" request bodies.

``` js
request.post('/upload')
  .field('foo', 'bar')
  .end(callback);

request.post('/upload')
  .field({ foo: 'bar', baz: 'qux' })
  .end(callback);
```

@param {String|Object} name name of field
@param {String|Blob|File|Buffer|fs.ReadStream} val value of field
@param {String} options extra options, e.g. 'blob'
@return {Request} for chaining
@api public