Make rep.lua only load default plugins if no rcfile is found

[hoelzro]

To address #47:

0.7

• [x] Move rlwrap functionality into its own plugin
• [x] Print a warning if an rcfile is found and repl.quiet_default_plugins is not set to true
  • [x] Allow plugins to return values, and have those values be propagated through loadplugin
  • [x] Print the warning
• [x] Add a method to prevent the loading of a particular plugin

  0.8

• [x] Change rep.lua to only load plugins if no rcfile is found
• [x] Deprecate plugin suppression

  0.9

• [ ] Remove plugin suppression