URL checker

If the upstream vendor has an URL that redirects to the latest version of the application, you can add something like the following to check and update the URL for the latest version:

"x-checker-data": {
    "type": "rotating-url",
    "url": "http://example.com/last-version",
    "pattern": "http://example.com/foo-v([0-9.]+).tar.gz"
}

The version number for the latest version can be detected in two ways:

• If the filename ends with .AppImage, the version number is extracted from the AppImage. (It is run in a bwrap sandbox.)
• Otherwise, if "pattern" is specified in "x-checker-data", the given regular expression is matched against the full URL for the latest version, and the first match group is taken to be the version. (This follows the convention used by debian/watch files.)