Mirrors can now be declared using the `mirror` method which works similar to
`depends_on` and takes the same arguments as `url`.

The formula class now has a public `fetch` method that cycles through the
mirror list if the downloader for the primary URL throws a `DownloadError`.
Other brew commands, like brew-fetch, also benefit from mirror support by using
this method.

Closes Homebrew/homebrew#7574.