Improving experimental feature handling

[rahul286]

• ee stack switches for dependency management e.g. ee stack install --hhvm, ee stack install --pagespeed
• Delayed dependency installation. e.g. do not install hhvm till ee site create example.com --hhvm is called.
• A clear warning and prompt e.g "HHVM is experimental feature and it may not work with all plugins all your site. You can disable it by passing hhvm=off later. Do you wish to enable HHVM now for example.com? Type "yes" to continue. [no] __". This prompt can be skipped by adding --experimental flag to original command.

Ref: http://community.rtcamp.com/t/installation-process-for-hhvm-fails-causing-ee-installation-to-halt/4605

[gau1991]

Fixed in EasyEngine 3.2.1