make handleAnalyticsEvents attribute optional (again)

[pietrop]

The presence of Keyboard shortcuts seems to force to have analytics function on (see 50da859 , ae20053) it be good to make it optional.

see README#usage--production

Note: in current version you need to add handleAnalyticsEvents function to be able to use keyboard shortcuts. But you can also just add a placeholder function if you don't wish to implement analytics

<TranscriptEditor
 transcriptData={someJsonFile}
 mediaUrl={"https://download.ted.com/talks/KateDarling_2018S-950k.mp4"}
 ...
 handleAnalyticsEvents = {()=>{console.log('handleAnalyticsEvents')}}
/>