added nocomments and verbatim passes

[benedictbrown]

nocomments pass strips comments but preserves whitespace. verbatim preserves comments and whitespace (as opposed to exact that strips whitespace). Preserving whitespace can be useful, especially in CS1 classes.

[cmlsharp]

I think the more passes we add, the less sense it makes to run every pass by default. Mainly for usability reasons (e.g. the differences between 'exact' and 'verbatim' might be confusing to a new u
user), but a little for performance I guess as well.

I added a bit of code to allow passes to be marked as default or not (via a class variable). There's probably a discussion worth having over what passes should be enabled by default but for now I just marked the three existing ones as default. The new passes can still be enabled at runtime via -p. Does this seem reasonable?