Allow state override

[mtdowling]

Make CodeWriters intercept Object instead of String

This commit makes CodeWriter section interceptors deal with Object
instead of String. This allows for the toString() method of the
intercepted text to be wrapped by classes that extend from CodeWriter in
order to determine if any side effects from the original content should
take effect or be discarded. The general idea is that if toString() was
called on the original content of an intercepted section, then original
contents are also going to be written to the CodeWriter, meaning any
side effects should also occur (for example, adding imports).

Allow pushState and popState to be overridden

When extending CodeWriter, you might need to also track state like
imports in a corresponding stack. This is now possible since pushState
and popState can be overridden.