Overhaul ColumnFileReader

[NebelNidas]

• Adds stacked mark support.
• Removes some weird inconsistencies where nextCol("") would return false even though nextCol() would return exactly that string. Now consistently returns null when encountering or already being at EOL, and an empty string when the column or line is empty.
• Tries to properly mark EOF too alongside EOL, preventing situations where the reader gets stuck at EOL at the end of the file but never toggles EOF to true.
• Adds peek support (reading without consuming, kinda like nextCol(String) already worked when the string didn't match).
• Adds support for custom indentation characters.
• Improves documentation.