Besides fixing https://github.com/prometheus/prometheus/issues/805 by
making the entire externally reachable server URL configurable, this
adds tests for the "globalURL" template function and makes it easier to
test other such functions in the future.
This breaks the `web.Hostname` flag (and introduces `web.external-url`).
This flag is likely only used by few users, so I hope that's
justifiable.
Fixes https://github.com/prometheus/prometheus/issues/805
Version information is determined at build-time and thus there is
no need to pass it down from main. In its own package it can
be used from various other packages.
