Add the ability to customize/disable Xcode command

This adds a `--no-open` cli flag as well as an `xcode_command` key to
the liftoffrc.

`--no-open` will let the user completely disable liftoff's ability to
automatically open Xcode after successfully generating a project

`xcode_command` lets the user customize the command used to open Xcode.
The contents of this key are executed arbitrarily, so allow the user to
customize their development process by using beta versions of Xcode,
AppCode, Vim, Emacs, etc instead of using the default version of Xcode.
Setting this key to `false` will disable this behavior completely.

Fixes #166