Ignore code between `#if false` ... `#endif`?

[Sjord]

E.g.

#if false
    this is a fairly common pattern in C# to disable code, but can also be used for comments or anything else.
#endif

Currently, this grammar basically ignores preprocessor directives. The parser is not aware of preprocessor variables, so it can't really do any better. However, in case of #if false we can be certain that the code shouldn't end up in the parse tree.

This introduces an extra special case for preprocessor directives, but it would improve parsing results.

[damieng]

I'd rather try and get #189 tackled which would cover this.