Labyrinth: Add file save / load

[graninas]

Add a possibility to save and load the game from a text file.

Commands:
save <file_name>
load <file_name>

[graninas]

Tips on the task.

• Serialization to and from text can be done using either of the two approaches:
  • Show / Read
  • ToJSON / FromJSON (preferred)
• There should be a serializable data type which contains all the information about the game to be saved / loaded. This data type should include:
  • Labyrinth itself
  • Player position
  • Treasure position
  • Game state