Notifier Class Reference

Notifiers send build system events to a front end or may implement a front
end themselves, displaying warnings and errors for a user.

This is different from a logger in a few ways:
 * The structure of the events are defined by this interface.
 * A "progress" level is included allowing signaling completion status to
   users.
 * It's tailored to providing events from a build system.

The structure of a message is a dict with a 'type' key. The type key
determines the remaining keys as follows:
  type       | description and remaining keys
  ---------- | ------------------------------
  info       | A simple message. The 'message' key contains the message
  debug      | Another simple message; this one is less useful when compiles
             | are working. Again, the 'message' key contains the message
  progress   | A progress indicator, which may include progress as a
             | percentage. The action key includes what action was taken to
             | make this progress, the file key what file was used to make
             | this progress, and the percent key, when present, indicates
             | how far along the build is.
  tool_error | When a compile fails, this contains the entire output of the
             | compiler.
  var        | Provides a key, in the 'key' key, and a value, in the 'value'
             | key, for use in a UI. At the time of writing it's used to
             | communicate the binary location to the online IDE.