Response function to help attach/download files in the Public directory

[Badgerati]

Add a new helper response function to attach a file to the response that can be downloaded; files should come from the Public directory, much like normal content requests.

ie, doing the following in a route would start a download for the public/downloads/install.exe file:

route get '/installer' {
    param($s)
    attach 'downloads/install.exe'
}