extemopre.el fixes

`extempore-repl` wasn't working for me, so I made a few improvements:
- If the user doesn't manually specify `--runtime` the defun adds it in relative to `user-extempore-directory`.
- Everything still works if user-extempore-directory includes "~/" or if the user forgot the trailing
slash.
- `extempore-repl` now polls the comint buffer every second and as soon as its ready it gets on with it.
- If either/both the process buffer or the REPL buffer are missing, they'll get spun up.
- If the user asks for `extempore-repl` and the repl is already running, switch to it just the same.