Add getCurrentTime to iOS

[liamse]

Sometimes it is required to get current time of recording in a moment. This method return an promise that can use like this:

import {AudioRecorder} from 'react-native-audio';
// ...
AudioRecorder.getCurrentTime().then(currentTime => {
    if (currentTime) {
        console.log('current Time', currentTime);
    } else {
        console.log('The recorder is not in progress');
    }
})