REPL – Add "magic" mode detection, persistent history support

This PR adds two public API features: the ability to configure the maximum history size of readline, and the ability to change the "mode" a REPL is started in – where "mode" may be "strict mode", "sloppy mode", or "magic / auto-detect mode". Auto-detect mode catches SyntaxErrors due to V8's lack of support for certain constructs (classes, let, and const) outside of strict mode, and retries the utterance in strict mode. User REPLs default to "sloppy mode" as before.

It also adds an internal module that controls the built-in CLI REPL. This module augments the REPL instance with the following features:

1. If NODE_REPL_HISTORY is set to a file path, it will attempt to use that file to back up history in JSON format. This punts on the painful issue of trying to determine a user's home directory in a cross-platform way.
2. If NODE_REPL_HISTORY is not set, pressing "up" (or "ctrl p") in the CLI REPL when there are no other lines in history will print instructions on how to set up persistent history support. It will only print this message once per session. The goal is to capture intent and communicate a way to enable support.
3. If NODE_REPL_MODE is set, it will start the REPL in that mode. The options are strict, sloppy, and magic. If not given, the REPL will start in "magic" mode. The goal is to reduce surprise when folks get the new V8 and try to write a class in the REPL immediately.
4. If NODE_REPL_HISTORY_SIZE is set and is a Number, it will set the maximum history to that size.

None of these features interact in any way with the REPL module, and are strictly additive for the CLI only.

Putting this up for an early review to make sure the features (and approach) are acceptable.

R=@rvagg

history example

modes example

First, "sloppy" mode (default as of old iojs), then "strict" mode, then "magic" mode.