Altered handling of --end-of-week

Used context argument in switch_message()

Added LaTeX templates ...

This required adding a separate Jinja2 environment, because LaTeX syntax
is incompatible with Jinja2.

Fully implemented --end-of-week/-e switch ...

(was --week-offset/-w)

Added initial code for --week-offset CLI switch

Preliminary code to refactor CLI error messages

Initialisation of ignored CLI args moved to cli.py

Fixed incorrect raising of PaperError

Major revamp of configuration handling ...

• Moved configuration file loading into Configuration class.
• Added config getter methods to Configuration.
• Updated and extended custom exception types.
• Restructured error handling.
• Factored out app name.

Fixed broken “offsets” for INFO202/S2

Deleted obsolete comment

Converted load_config() into Configuration class

Removed reference to PROG constant

Moved load_config() into config.py

Added configuration defaults ...

The app nows looks for configuration in ~/.config if no configuration
file is explicitly specified. If the default configuration isn’t found,
a default configuration file is copied into ~/.config.

Refactored program name

Renamed module