symbols for certain keys
⌘ == command
⌥ == option
⌃ == control
fn == function
⇧ == shift
⇪ == caps Lock
⌫ == delete
⌦ == 向前删除键（fn⌫）
↩ == Return/Enter
↑ == 上箭头
↓ == 下箭头
← == 左箭头
→ == 右箭头
⇞ == page up（fn↑）
⇟ == page down（fn↓）
home == fn←
end == fn→
⇥ == 右制表符（tab）
⇤ == 左制表符（⇧tab）
⎋ == escape (esc)
⏏ == 电源开关键
⌘⇧. 显示隐藏文件
⌘H Hide the windows of the front app. To view the front app but hide all other apps, press ⌘⌥H.
⌘W Close the front window. To close all windows of the app, press ⌘⌥W.
参考资料：
http://www.jianshu.com/p/735ba08df1fa
https://support.apple.com/en-us/HT201236
