prompt: Add the ability to add new shortcuts to awful.prompt

Example:

awful.prompt.run({ prompt = "Run: ", hooks = {
    {{         },"Return",function(command)
         local result = awful.util.spawn(command)
         mypromptbox[mouse.screen].widget:set_text(type(result) == "string" and result or "")
         return true
    end},
    {{"Mod1"   },"Return",function(command)
         local result = awful.util.spawn(command,{sticky=true})
         mypromptbox[mouse.screen].widget:set_text(type(result) == "string" and result or "")
         return true
    end},
    {{"Shift"  },"Return",function(command)
         local result = awful.util.spawn(command,{ontop=true,floating=true})
         mypromptbox[mouse.screen].widget:set_text(type(result) == "string" and result or "")
         return true
    end}
    }
},...)

Another old PR undusting