Waits for element to be clickable (by default waits for 1sec).
Element can be located by CSS or XPath.

```js
I.waitForClickable('.btn.continue');
I.waitForClickable('.btn.continue', 5); // wait for 5 secs
```

@param {CodeceptJS.LocatorOrString} locator element located by CSS|XPath|strict locator.
@param {number} [sec] (optional, `1` by default) time in seconds to wait
[!] returns a _promise_ which is synchronized internally by recorder