Add compile warnings

[Kangaroux]

The compiler currently has error handling where it will raise a ParseError if it encounters some bad syntax and doesn't know how to proceed. However, there are times where it would be more appropriate to notify the user about something but still proceed with compilation.

For example, if the user writes a conditional comment but puts some text after the comment:

-#[if IE] like this

We can parse that line just fine, but the text is ignored so it serves no purpose. It would be nice to notify the user that it should be removed.