TS-1753: Add remap support to the cacheurl plugin

  - Update README for cacheurl plugin with examples
  - Allow specifying cacheurl.config location. This is done as the
    first parameter in plugins.config.
  - Allow cacheurl to be used as a remap plugin. This also requires
    that the config be stored either with the remap instance or
    the transaction continuation (depending on how the plugin is called)
    and that more than one configuration be able to be loaded. The
    configuration is now dynamically allocated as a result.
  - Convert README to restructured text (for later inclusion in
    official docs). Mention remap plugin invocation in the docs.