Introduce prefixes in `bindAll`

Make `slate.bindAll` accept an optional argument of prefix map.

```javascript
// simple mapping
slate.bindAll({ "esc:ctrl;alt" : S.op("relaunch") });

// with prefixes
slate.bindAll({ layoutKeys: "ctrl;cmd" }, {
  "l:#layoutKeys": laptop,
  "t:#layoutKeys": thunderbolt,
  "b:#layoutKeys": twoMonitor
});
```