Added optional additional headers to createPlayer config for FetchStreamLoader

[yyyar]

Hello! First of all, thanks for the awesome library!

This PR adds additional optional headers to createPlayer config for use in FetchStreamLoader.
For example, it is needed to specify 'Authorization' Basic Auth header to allow fetch FLV stream from auth protected URL.

Example

var flvPlayer = flvjs.createPlayer({
    isLive: true,
    type: 'flv',
    url: "https:/localhost/flv",
}, {
    enableWorker: false,
    deferLoadAfterSourceOpen:false,
    enableStashBuffer: false,

    // additional authorization header
    headers: {
        'Authorization': 'Basic ' + btoa(unescape(encodeURIComponent("admin:1111")))
    },
});

There is withCredentials property on dataSource but it won't work if you don't want auth popup to appear for the user on a page that contains player. That's because allowing adding 'Authorization' header manually would be beneficial.

Thank you!

[xqq]

Sorry for my late response.
It's a good idea and thanks for your contribution.

Before merging I have to update the declarations in config.js and flv.d.ts

[xqq]

Now it will also work in other browsers like FireFox, etc.

Merged, thanks!