Add a reader mode with more exceptions

[mmastrac]

Per conversation with @dweiss in another issue, we should probably have a reader mode where exceptions are more readily thrown for things like type cast exceptions.

[mmastrac]

My API suggestion is that we should set this on readers themselves at construction time, something like:

JsonReader.throwCastExceptions().from(...)

This would match the builder style of JsonWriter from https://github.com/mmastrac/nanojson/blob/master/src/main/java/com/grack/nanojson/JsonWriter.java#L154

[dweiss]

+1.