Object model

[julianhyde]

Add an object model for Quidem. This will allow applications to parse a Quidem file into a sequence of commands and sections, process those elements, and write out a new Quidem file.

The object model is in the style of a streaming push parser. The parser reads a document, and calls an appropriate method on interface EventHandler for each element (event). For parsing, the user writes their own implementation of EventHandler with a method to handle each type of event. For writing, Quidem provides an implementation of EventHandler that converts the events is receives into a document string.

[julianhyde]

Dev branch: https://github.com/julianhyde/quidem/tree/41-object-model