Prepared package for adding GM sheets.

- Improved test coverage in ``make_sheets.py``.
- Refactor file loading to make it more flexible.
- Added a stub ``gm.py`` example.

examples/gm.py

@@ -0,0 +1,10 @@
+"""This file describes game-manager notes.
+
+It's used for creating notes for the GM to keep track of various
+monsters, etc.
+
+"""
+
+dungeonsheets_version = "0.14.0"
+
+sheet_type = "gm"