allow passing a repository to createSource

[Travis Arnold]

Add first-class support for analyzing another repository:

import { createSource } from 'mdxts'

export const allPackages = createSource('https://github.com/souporserious/react-measure')

This would be equivalent to querying it locally:

import { createSource } from 'mdxts'

export const allDocs = createSource('react-measure')