Allow user to define `live-root-dir'.

Hey Sam, I hope everything is in order. (This is my very first time doing this (a pull request).)

BTW, I don't have any tests to give you with this.  I can only proffer a bald assertion that it works for me just fine, as I outlined in the forum message.  Is this the best way to update the code to have a user-defined `live-root-dir`?  I don't know.

Here's something to think about.  Will this change break any other script/code?  (Or more precisely, will moving the Emacs Live root to something other than `~/.emacs.d` break any extant code?)  A candidate for breakage, for instance, might be the script `packs/update-live-packs` which has hardcoded paths, like `~/.emacs.d/packs/live`.  (I went to the root of emacs-live and executed a `grep -R '~/.emacs.d' *` to reveal this.  There are many more grep hits, but most if not all may not be problems at all.)

Thanks! —Rick