Implement autoreloading on module change

[dbkaplun]

See otra-vez for a Node shell that does this.

[danielgtaylor]

This looks pretty cool. +1 for adding this to nesh, and it could be a built-in plugin so long as it doesn't introduce a bunch of dependencies.

Question - if I run otra file1.js and in file1 I require('file2') and then update file2 - does otra pick up the change?

[apaleslimghost]

Otra currently only hooks into require calls directly in the REPL, but that's something we could add.

Edit: see quarterto/otra-vez#3.

Currently otra-vez only exports the REPL, I've filed quarterto/otra-vez#4 to split it up. The only dependency the require portion has is clear-require, which is pretty small.