Make interim login URL filterable

[jfarthing84]

Currently, there is no clean way to detect and filter the interim login URL, short of using the clean_url filter. Obviously, this is not ideal. Plugins that modify the login URL, like my plugin, Theme My Login, would definitely benefit from a direct filter on this.

[jfarthing84]

Add interim_login parameter to wp_login_url().

[jfarthing84]

Alternatively, and actually a better approach, adds another parameter to wp_login_url(), namely $interim_login as a flag to handle creating the interim login URL wthin wp_login_url().