Support class and instance initializers

This adds support for class and instance initializers.

For example

````
writer.emitPackage("com.example")
    .beginType("com.example.Person", "class", PUBLIC | FINAL)
    .emitField("String", "firstName", PRIVATE | STATIC)
    .beginInitializer("static")
    .emitStatement("firstName = \"John\"")
    .endInitializer();
````

Would produce

````
package com.example;

public final class Person {
  private String firstName;
  static {
    firstName = "John";
  }
}
````