Add pre_get_home_url hook for plugins

[technosailor]

We need to allow plugins to "do stuff" in pre_get_home_url. In profiling today, we wrote some code that, if we have this hook, would allow a reduction of computations from x (in our case, 4000) down to 1 using a pseudo-cache technique.

While that code isn't core-worthy, having a hook to allow plugin modification would incredibly enhance the ability to make WP more efficient on scale.

adds pre_get_home_url action.

Can we hear more about the use case?

If we were to do this, I'd think we'd do it for all _url() functions.