1. A computer system for, upon request of a requestor, downloading custom code for execution within a host application, the computer system comprising: a component that, when requested by the requester, downloads a manifest for the custom code that specifies components of the custom code and includes information describing how the custom code is to be used; a component that reports to the requestor progress during the download of the manifest so that additional processing can be performed by the requestor when downloading the manifest; a component that downloads the custom code in accordance with the downloaded manifest; a component that verifies trust of the custom code by invoking functionality provided by the requester; a component that reports to the requestor progress during the download of the custom code so that additional processing can be performed by the requestor when downloading the custom code; and a component that uninstalls the custom code when requested by the requestor.