Vagrant::UI::Basic#ask: support a noecho option for stdin

Support using "ask" without echoing back stdin for passwords and etc.

Relies on io/console and http://ruby-doc.org/stdlib-2.0.0/libdoc/io/console/rdoc/IO.html#method-i-noecho.

However io/console is not supported in 1.8.7 or 1.9.2.