Add registerPath config variable

[leevigraham]

I know Craft does not supply a built in registration form like login, forgot password etc… but it does support the ability to create your own.

It would be nice to have the ability to set registerPath in config just like loginPath, logoutPath, setPasswordRequestPath for sites that do have registration forms. This would mean a custom config file does not have to be created for a single setting and devs can access all of these account related urls in the same manner.