FIXED: Did not always ignore changes made by program.
Group together classes for handling file formats.
Move reading plain text files into separate class.
Unify API for file format readers.
Simplify code for reading file formats in document.
Move type check into format readers.
Group logic for handling file formats.
Add wrapper for libzip.
Add basic Office Open XML support.
Clean up document writer class.
Add custom OpenDocument Text writer.
Switch to a single unit indent.
Add setting tab width in theme. Closes #113.
Update included libzip version to 0.11.1.