extempore.el: Be more forgiving about user-extempore-directory

- shell expansion
- missing trailing slash
extempore.el: pass --runtime  if not provided when running extempore inferior
extempore.el: harden extempore-repl and wait minimally

- Do the right thing if either/both extemoreor repl buffer are missing
- As soon as the repl is available, exit the wait loop
- Report to user.
- Switch to REPL if it's (already) there.
extempore.el: fix message to match keybinding switch to C-c
Merge branch 'elisp_fixes' of https://github.com/giorashh/extempore

* 'elisp_fixes' of https://github.com/giorashh/extempore:
  extempore.el: fix message to match keybinding switch to C-c
  extempore.el: harden extempore-repl and wait minimally
  extempore.el: pass --runtime  if not provided when running extempore inferior
  extempore.el: Be more forgiving about user-extempore-directory