v0.4.0

• BREAKING: Listen on localhost instead of listening publicly by default.
• BREAKING: We no longer set the Access-Control-Allow-Origin HTTP header to
  *. This was previously done for convenience but we've decided to prioritize
  security here by removing this default. You can still add this header to any
  route you want to be accessible from other origins.
• BREAKING: Listen on a random port by default instead of always on 8000. This
  can be controlled using the port parameter in run(), or by setting the
  plumber.port option.
• BREAKING: Removed PlumberProcessor class and replaced with a notion of
  hooks. See registerHook and registerHooks on the Plumber router.
• BREAKING: addGlobalProcessor method on Plumber routers now takes a list
  which are added as hooks instead of a Processor. Note that sessionCookie
  has also been updated to behave accordingly, meaning that the convention of
  pr$addGlobalProcessor(sessionCookie("secret", "cookieName")) will continue
  to work for this release.
• BREAKING: sessionCookie now returns a list instead of a Processor. Note
  that addGlobalProcessor has also been updated to behave accordingly,
  meaning that the convention of
  pr$addGlobalProcessor(sessionCookie("secret", "cookieName")) will continue
  to work for this release.
• DEPRECATION: Deprecated the addAssets method on Plumber routers. Use
  PlumberStatic and the mount method to attach a static router.
• DEPRECATION: Deprecated the addEndpoint method in favor of the handle
  method for Plumber routers. Removed support for the processors, params,
  and comments parameters are no longer supported.
• DEPRECATION: Deprecated the addFilter method on Plumber routers in favor
  of the new filter method. Removed support for the processor parameter.
• DEPRECATION: Deprecated the addGlobalProcessor method on Plumber routers.
• The undocumented setDefaultErrorHandler method on Plumber routers now takes
  a function that returns the error handler function. The top-level function
  takes a single param named debug which is managed by the debug parameter
  in the run() method.
• Added support for OPTIONS HTTP requests via the @options annotation.
• Add support for entrypoint.R when plumb()ing a directory. If this file
  exists, it is expected to return a Plumber router representing the API
  contained in this directory. If it doesn't exist, the behavior is unaltered.
  If both plumber.R and entrypoint.R exist, entrypoint.R takes precedence.
• plumb() the current directory by default if no arguments are provided.
• Added a debug parameter to the run method which can be set to TRUE in
  order to get more insight into your API errors.