[Feature] Event listeners

**Is your feature request related to a problem? Please describe.**
I'm making a browser extension with this module, and since there's a loading time, I need a way to inform the user when FFmpeg is usable.

**Describe the solution you'd like**
Have either:
```ts
ffmpeg.on("load", () => {});
```
or
```ts
ffmpeg.onLoad(() => {});
```
As well as a way to tell that FFmpeg has become inactive, i.e., finished the series of operations set in the code.

**Describe alternatives you've considered**
As a temporary solution, I can set a custom logger function and run a RegExp match against it, which will use `chrome.runtime.sendMessage()` to update the client that FFmpeg is ready.

**Additional context**