Adding support for multiple combos syntax

I've added support for "multiple combos" as alternative of many combox separated by slash.
You can now write
```javascript
jwerty.is("cmd+[a,b,c]", event)
jwerty.is("cmd+shift+[a,b,c]", event)
jwerty.is("[ctrl, cmd]+a", event)
```
instead of

```javascript
jwerty.is("cmd+a/cmd+b/cmd+c", event)
jwerty.is("cmd+shift+a/cmd+shift+b/cmd+shift+c", event)
jwerty.is("cmd+a/ctrl+a", event)
```

And next example demonstrates the power of new syntax.

Old style:
```javascript
jwerty.is("backspace/backspace+ctrl/backspace+cmd/backspace+alt/delete/delete+ctrl/delete+cmd/delete+alt", event)
```

New style:

```javascript
jwerty.is("backspace/backspace+[cmd,ctrl,alt]/delete/delete+[cmd,ctrl,alt]", event)
```

This syntax works like abbreviations and my code just expands it into full combo.
Everything from old syntax (such as `[a-z]` or `[0-9]`) works as usual.