x/term: cross-platform key codes

[techtonik]

Add cross-platform key identifiers that can be used to detect keypresses for user shortcuts regardless of keyboard layouts.

More info:

• https://stackoverflow.com/questions/8737566/rolling-ones-own-keyboard-input-system-in-c-c
• https://hg.libsdl.org/SDL/file/default/include/SDL_scancode.h
• http://www.usb.org/developers/hidpage/Hut1_12v2.pdf

About names - let's keep them consistent with SDL:

• KeyUp
• KeyPageUp

[techtonik]

https://github.com/yakshaveinc/go-keycodes

[techtonik]

Discussion https://groups.google.com/forum/#!topic/golang-nuts/dOBYOEuX-Ss