vex-js: added `defaultOptions`
vex-js: added `defaultOptions`

To support
```js
vex.defaultOptions.className = 'vex-theme-os';
```
chrootsu/DefinitelyTyped,minodisk/DefinitelyTyped,GiedriusGrabauskas/DefinitelyTyped,abbasmhd/DefinitelyTyped,isman-usoh/DefinitelyTyped,AbraaoAlves/DefinitelyTyped,zuzusik/DefinitelyTyped,benliddicott/DefinitelyTyped,zuzusik/DefinitelyTyped,abbasmhd/DefinitelyTyped,AbraaoAlves/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,markogresak/DefinitelyTyped,laurentiustamate94/DefinitelyTyped,magny/DefinitelyTyped,benishouga/DefinitelyTyped,minodisk/DefinitelyTyped,QuatroCode/DefinitelyTyped,borisyankov/DefinitelyTyped,zuzusik/DefinitelyTyped,rolandzwaga/DefinitelyTyped,georgemarshall/DefinitelyTyped,isman-usoh/DefinitelyTyped,amir-arad/DefinitelyTyped,benishouga/DefinitelyTyped,ashwinr/DefinitelyTyped,alexdresko/DefinitelyTyped,benishouga/DefinitelyTyped,AbraaoAlves/DefinitelyTyped,jimthedev/DefinitelyTyped,one-pieces/DefinitelyTyped,aciccarello/DefinitelyTyped,nycdotnet/DefinitelyTyped,dsebastien/DefinitelyTyped,arusakov/DefinitelyTyped,arusakov/DefinitelyTyped,QuatroCode/DefinitelyTyped,chrootsu/DefinitelyTyped,GiedriusGrabauskas/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,georgemarshall/DefinitelyTyped,arusakov/DefinitelyTyped,magny/DefinitelyTyped,alexdresko/DefinitelyTyped,borisyankov/DefinitelyTyped,aciccarello/DefinitelyTyped,AgentME/DefinitelyTyped,aciccarello/DefinitelyTyped,jimthedev/DefinitelyTyped,AgentME/DefinitelyTyped,jimthedev/DefinitelyTyped,ashwinr/DefinitelyTyped,nycdotnet/DefinitelyTyped,rolandzwaga/DefinitelyTyped