Refactor line continuation handlers

[max99x]

Each engine has a method that decides whether a given command should be passed to the evaluator or continued to the next line, and in the latter case, whether there should be any change in indentation. Right now a lot of code for doing this is shared between the engines. Once we have all or most of the engines added, we should factor out the common code into a small util library.

[max99x]

Removed from launch milestone. As nice as it would be to have this, it's not really critical to launching.