repl: Assignment of _ allowed with warning

This commit addresses https://github.com/nodejs/node/issues/5431 by
changing the way that the repl handles assignment to the global _
variable.

Prior to this commit, node sets the result of the last expression
evaluated in the repl to `_`. This causes problems for users of
underscore, lodash and other packages where it is common to assign
`_` to the package, e.g. `_ = require('lodash');`.

Changes in this commit now result in the following behavior.

- If unassigned on the repl, `_` continues to refer to the last
  evaluated expression.
- If assigned, the default behavior of assigning `_` to the last
  evaluated expression is disabled, and `_` now references whatever
  value was explicitly set. A warning is issued on the repl -
  'expression assignment to _ now disabled'.
- If `_` is assigned multiple times, the warning is only displayed once.
- When `.clear` is executed in the repl, `_` continues to refer to its
  most recent value, whatever that is (this is per existing behavior).
  If `_` had been explicitly set prior to `.clear` it will not change
  again with the evaluation of the next expression.

PR-URL: https://github.com/nodejs/node/pull/5535
Fixes: https://github.com/nodejs/node/issues/5431


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>