Serve behind route prefix

[kwilt]

This new parameter will allow for serving the landing page when using --web.external-url as an argument with many exporters.

I added this as it is needed for this pull request: prometheus/snmp_exporter#1335

So in practice it will look something like this:

landingConfig := web.LandingConfig{
        // config stuff goes here ...
		}
                                                                      // new parameter
		landingPage, err := web.NewLandingPage(landingConfig, *routePrefix != "")
		if err != nil {
			logger.Error("Error creating landing page", "err", err)
			os.Exit(1)
		}
		http.Handle(*routePrefix, landingPage)

[kwilt]

Okay, I think I've made the changes you requested

[SuperQ]

Looking good so far, thanks!

[kwilt]

Anything else you'd like fixed/added to this? 😄

[SuperQ]

Thanks!