First implementation of a watcher

This is just to open the discussion thread of the watch mode.
This should allow to detect external .minni file changes and reload them in the REPL to avoid de-sync.
This first commit uses a setInterval to watch for stats.mtime instead of fs.watch because it did not work.

2 things to work on:

- check the fs.watch API
- make the @reload() returned string appear in the REPL even when called from the fileChanged callback