feat: replace/extend default HTTP headers with custom ones

[Endebert]

Added a new (optional) argument to the pipe() function that adds the ability to replace the default HTTP headers for the request, or add custom ones.

Includes additional test and updated README.md.

This covers the use-case for #1. However, my PR adds this functionality using a new argument, i.e. it's not a breaking change.

In the meantime, my modified version is published as "@tillhub/node-ssestream": https://www.npmjs.com/package/@tillhub/node-ssestream

[soyuka]

this should be added via the constructor, see my fork at https://github.com/soyuka/ssestream

[Endebert]

@soyuka Thanks for the info. Could you please try your luck & create a PR with your changes? Maybe you have more luck than us, though I don't have high hopes.

[soyuka]

@Endebert if this project is maintained the maintainers will definitely have a look, if not you should use something that is maintained :). Also you have maf's alternative that is also widely used since longer: https://github.com/mafintosh/event-source-stream/.

In my fork I took the liberty of merging a few resouces that I found over the web:
https://github.com/EventSource/node-ssestream/blob/master/README.md
https://gist.github.com/gitawego/8ef33ec7d895498f2eedd95eafa835bb
#2

[Endebert]

@soyuka Thanks for all the info :)