Add "Accept terms of use" prompt/message when downloading module

If a module has a "terms" field in its manifest entry, the value is assumed to be a HTTP URL to a web page containing information about terms of use for the module. A message should be printed prompting the user to read and accept the terms.

Possibly just a message, e.g. By downloading and using this module you are agreeing to the terms of use outlined at http://..../, rather than forcing the user to respond to a prompt.