Fix the default ini value for "arg_separator.input" to be "&"

Summary: This fixes #3256

Closes https://github.com/facebook/hhvm/pull/3257

Reviewed By: @JoelMarcey

Differential Revision: D1452078

Pulled By: svcscm
Implement a proper hack-mode for Emacs

Summary: This mostly supercedes the existing `hack-for-hiphop.el` "integration" that already exists. This is my first time doing anything major with Emacs, so it ~~might~~ certainly needs some more work. A few issues I can think of immediately are that it doesn't highlight XHP expressions and it likely won't indent them properly. Indentation is hard though, so I'm leaving that until later.

##Features

* Highlighting of various Hack-specific constructs, including Hack-specific keywords.
* Better integration with the typechecker by *not* setting global keybindings unlike the current integration, instead creating mode-specific bindings.
* Integration with the completion service provided by the typechecker. Includes integration with the `auto-complete` Emacs extension for intellisense-style autocompletion.
* Doesn't depend on an existing `php-mode` of which there are atleast 3 different ones of various completeness and complexity.

Closes https://github.com/facebook/hhvm/pull/3209

Reviewed By: @JoelMarcey

Differential Revision: D1440799

Pulled By: svcscm