(MAINT) Make user repl config dynamic

This commit modifies `user_repl.clj` and the sample `user.clj`
file to use a function instead of a var for the user-provided
config.

Prior to this, the config was read from disk as a var when the
REPL was first launched, and never re-loaded after that.  This
meant that doing a `reset` would not pick up changes you might
have made to the config files on disk; you needed to restart
the REPL to pick up those changes.

Now, the user config is acquired via a function call, which
means that your config files will be reloaded each time you
call `reset`.
the code in there shows how to do load-config, which just returns a map, and you could assoc anything you wanted into that map before returning it.  you could choose to point load-config at the sample config file if you just wanted the defaults.  for now i think this should just be something that individual users decide what they want to do in their environment.  if you customize your own personal user.clj and think that your changes are nice enough to want to share with others, you can open a new PR to modify the sample file.