Provide configurable path prefix

[miri64]

We integrated the mastodon feed of our open source project on our website (see https://www.riot-os.org/) behind a reverse proxy. In previous versions of mastofeed we could just rewrite the path and it works. However, some updates to mastofeed make this impossible. Here is the, admittedly somewhat hacky, way how I solved this possible for now. In case you do not want to merge this: Maybe it helps someone else who has similar problems.

To test, one can add e.g. the following to a Caddy site:

	handle /mastofeed/* {
		uri strip_prefix /mastofeed
		reverse_proxy localhost:8080
	}

	rewrite /.well-known/webfinger /mastofeed{uri}

[fenwick67]

Thank you for this, and I'm glad to see someone else could make use of this project.

I'll give this a spin when I have some free time.