ReadableStream parse input

[dbushell]

For issue #1 - this PR allows an optional ReadableStream input to the parse function instead of a URL.

Current URL example:

const parser = parse('https://feed.syntax.fm/rss');

Stream example:

const response = await fetch('https://feed.syntax.fm/rss');
const parser = parse(response.body);

I've updated the Deno example to use both.

[dbushell]

fixes #1