Add http_status migrate_condition plugin

Problem/Motivation

I've always wondered if a condition that gets the http status code of a url would be a good idea. If the download process plugin gets a 404, some wierd things can happen. It might be nice to be able to dance around that a bit. There are probably good use cases.

Proposed resolution

Do it!

Examples:

 * 1. Skip the row if a file url gives 4xx or 5xx response.
 *
 * @code
 * process:
 *   _file_trouble:
 *     plugin: skip_on_condition
 *     method: row
 *     source: my_source_url
 *     condition: not:http_status
 * @endcode
 *
 * or simply skip processing a file field before download fails:
 *
 * @code
 * process:
 *   path_to_file:
 *     -
 *       plugin: skip_on_condition
 *       method: process
 *       source: source_url
 *       condition: not:http_status
 *     -
 *       plugin: download
 *       source:
 *         - source_url
 *         - destination_uri
 * @endcode
 *
 * 2. Store the teapot status.
 *
 * @code
 * process:
 *   field_is_a_teapot:
 *     plugin: evaluate_condition
 *     source: my_source_url
 *     condition:
 *       plugin: http_status
 *       code: 418
 * @endcode
 *
 * or equivalently
 *
 * @code
 * process:
 *   field_is_a_teapot:
 *     plugin: evaluate_condition
 *     source: my_source_url
 *     condition: http_status(418)
 * @endcode
 *
 * or even
 *
 * @code
 * process:
 *   field_is_a_teapot:
 *     plugin: evaluate_condition
 *     source: my_source_url
 *     condition:
 *       plugin: http_status
 *       code:
 *         - 418
 * @endcode

Remaining tasks

User interface changes

nope

API changes

http_status

Data model changes

nope