Support for low-level agent access

[jkrems]

This allows user to directly access the protocol by exposing the agent proxies inside of the repl context. Because of the existing promise magic, things like the following "just work":

> ./cli.js examples/empty.js
# ...
debug> Debugger.getScriptSource({ scriptId: '64' })
{ scriptSource: '(function (exports, require, module, __filename, __dirname) { \n});' }
debug> _.scriptSource
'(function (exports, require, module, __filename, __dirname) { \n});'

[jkrems]

/cc @joshgav I think you brought this up as a useful feature in the meeting yesterday.