Add IgnoreUnknownColumns to CsvFileDescription

Adds feature discussed in #9. 

- Modify `article.htm` to document the changes
- Add unit test to make sure the new functionality works
- Add ToString() method in `TypeFieldInfo` class for better debugging

**A few important things:**

- There was an existing (undocumented) `IgnoreMissingColumns` field, used mainly for `UseFieldIndexForReadingData`. It was renamed to `IgnoreUnknownColumns`, since the purpose of `IgnoreMissingColumns` is covered by `IgnoreUnknownColumns` now, to avoid having duplicate fields.
- The entire code probably needs to be refactored at some point in the future, so more unit tests will be required. If we keep adding properties to the `CsvFileDescription` class, the code might become kind of an spaguetti one.