Allow to customise nitter instance to expose rss endpoints for twitter

[SineAnkama]

Since we have made a POC for an RSS feed for Twitter accounts, I'm open a PR here.

First of all thanks a lot for your work on nitter!

So this PR allows you to launch a custom instance of nitter to have a nitter API that expose an RSS feed to Twitter.

I don't know if this is relevant or not, but here is the list of our changes (and I admit that this is the first time I've worked with nim)

In any case, don't hesitate if you have any comments or questions (Or if this PR isn't relevant don't hesitate to close it)