[FEATURE] Support for event on Windows/Command/Option/Alt key presses

**Is your feature request related to a problem? Please describe.**
I cannot see definitions for several special keys in: https://docs.rs/cursive/0.16.3/cursive/event/enum.Key.html

* Windows
* Alt
* Command (⌘)
* Option (⌥)
* Function keys 13-19 (available on some keyboards, such as the Apple Magic Keyboard)

**Describe the solution you'd like**
All these keys should be available to create key bindings on for software using cursive/cursive-core.

**Describe alternatives you've considered**
A more limited set of key binding options is the only alternative to adding this feature I can think of.