Add warnings for bad coding patterns

CodeWorld could add its own warnings to those from the compiler, when it detects bad coding patterns that students often adopt. Some examples:

* Lots of duplicate code
* Lines longer than a certain length
* No spaces around an operator
* Lots of variables with the same name but different numbers after it, like `bird`, `bird1`, `bird2`, etc.
* Nesting that is too deep

This could be a gentle nudge to suggest that there's a better way.