Add .isatty() method to StdReader

StdWriter has .isatty(). StdReader can trivially vend the same function,
and someone asked today on IRC how to call isatty() on stdin.
auto merge of #14315 : kballard/rust/stdreader_isatty, r=alexcrichton

StdWriter has .isatty(). StdReader can trivially vend the same function,
and someone asked today on IRC how to call isatty() on stdin.